diff --git a/src/asm/parser.y b/src/asm/parser.y
index d051685e..275e4585 100644
--- a/src/asm/parser.y
+++ b/src/asm/parser.y
@@ -99,6 +99,10 @@
 
 	// REG_AF == REG_SP since LD/INC/ADD/DEC allow SP, while PUSH/POP allow AF
 	enum { REG_BC, REG_DE, REG_HL, REG_SP, REG_AF = REG_SP };
+	// Names are not needed for AF or SP
+	static char const *reg_tt_names[] = { "BC", "DE", "HL" };
+	static char const *reg_tt_high_names[] = { "B", "D", "H" };
+	static char const *reg_tt_low_names[] = { "C", "E", "L" };
 
 	// CC_NZ == CC_Z ^ 1, and CC_NC == CC_C ^ 1, so `!` can toggle them
 	enum { CC_NZ, CC_Z, CC_NC, CC_C };
@@ -380,6 +384,8 @@
 %type <int32_t> reg_ss
 %type <int32_t> reg_rr
 %type <int32_t> reg_tt
+%type <int32_t> reg_tt_no_af
+%type <int32_t> reg_bc_or_de
 %type <int32_t> ccode_expr
 %type <int32_t> ccode
 %type <Expression> op_a_n
@@ -2115,16 +2121,30 @@ sm83_ld_hl:
 		sect_ConstByte(0xF8);
 		sect_RelByte($5, 1);
 	}
+	| SM83_LD MODE_HL COMMA MODE_SP {
+		::error("LD HL, SP is not a valid instruction; use LD HL, SP + 0\n");
+	}
 	| SM83_LD MODE_HL COMMA reloc_16bit {
 		sect_ConstByte(0x01 | (REG_HL << 4));
 		sect_RelWord($4, 1);
 	}
+	| SM83_LD MODE_HL COMMA reg_tt_no_af {
+		::error(
+		    "LD HL, %s is not a valid instruction; use LD H, %s and LD L, %s\n",
+		    reg_tt_names[$4],
+		    reg_tt_high_names[$4],
+		    reg_tt_low_names[$4]
+		);
+	}
 ;
 
 sm83_ld_sp:
 	SM83_LD MODE_SP COMMA MODE_HL {
 		sect_ConstByte(0xF9);
 	}
+	| SM83_LD MODE_SP COMMA reg_bc_or_de {
+		::error("LD SP, %s is not a valid instruction\n", reg_tt_names[$4]);
+	}
 	| SM83_LD MODE_SP COMMA reloc_16bit {
 		sect_ConstByte(0x01 | (REG_SP << 4));
 		sect_RelWord($4, 1);
@@ -2197,13 +2217,20 @@ sm83_ld_a:
 ;
 
 sm83_ld_ss:
-	SM83_LD MODE_BC COMMA reloc_16bit {
-		sect_ConstByte(0x01 | (REG_BC << 4));
+	SM83_LD reg_bc_or_de COMMA reloc_16bit {
+		sect_ConstByte(0x01 | ($2 << 4));
 		sect_RelWord($4, 1);
 	}
-	| SM83_LD MODE_DE COMMA reloc_16bit {
-		sect_ConstByte(0x01 | (REG_DE << 4));
-		sect_RelWord($4, 1);
+	| SM83_LD reg_bc_or_de COMMA reg_tt_no_af {
+		::error(
+		    "LD %s, %s is not a valid instruction; use LD %s, %s and LD %s, %s\n",
+		    reg_tt_names[$2],
+		    reg_tt_names[$4],
+		    reg_tt_high_names[$2],
+		    reg_tt_high_names[$4],
+		    reg_tt_low_names[$2],
+		    reg_tt_low_names[$4]
+		);
 	}
 	// HL is taken care of in sm83_ld_hl
 	// SP is taken care of in sm83_ld_sp
@@ -2532,33 +2559,33 @@ reg_a:
 ;
 
 reg_tt:
-	MODE_BC {
-		$$ = REG_BC;
-	}
-	| MODE_DE {
-		$$ = REG_DE;
-	}
-	| MODE_HL {
-		$$ = REG_HL;
-	}
+	reg_tt_no_af
 	| MODE_AF {
 		$$ = REG_AF;
 	}
 ;
 
 reg_ss:
+	reg_tt_no_af
+	| MODE_SP {
+		$$ = REG_SP;
+	}
+;
+
+reg_tt_no_af:
+	reg_bc_or_de
+	| MODE_HL {
+		$$ = REG_HL;
+	}
+;
+
+reg_bc_or_de:
 	MODE_BC {
 		$$ = REG_BC;
 	}
 	| MODE_DE {
 		$$ = REG_DE;
 	}
-	| MODE_HL {
-		$$ = REG_HL;
-	}
-	| MODE_SP {
-		$$ = REG_SP;
-	}
 ;
 
 reg_rr:
diff --git a/src/asm/rpn.cpp b/src/asm/rpn.cpp
index 36009431..865816e7 100644
--- a/src/asm/rpn.cpp
+++ b/src/asm/rpn.cpp
@@ -582,7 +582,7 @@ void Expression::makeCheckBitIndex(uint8_t mask) {
 // Checks that an RPN expression's value fits within N bits (signed or unsigned)
 void Expression::checkNBit(uint8_t n) const {
 	if (isKnown()) {
-		::checkNBit(value(), n, "Expression");
+		::checkNBit(value(), n, nullptr);
 	}
 }
 
@@ -591,11 +591,23 @@ bool checkNBit(int32_t v, uint8_t n, char const *name) {
 	assume(n < CHAR_BIT * sizeof(int)); // Otherwise `1 << n` is UB
 
 	if (v < -(1 << n) || v >= 1 << n) {
-		warning(WARNING_TRUNCATION_1, "%s must be %u-bit\n", name, n);
+		warning(
+		    WARNING_TRUNCATION_1,
+		    n == 8 && !name ? "%s must be %u-bit; use LOW() to force 8-bit\n"
+		                    : "%s must be %u-bit\n",
+		    name ? name : "Expression",
+		    n
+		);
 		return false;
 	}
 	if (v < -(1 << (n - 1))) {
-		warning(WARNING_TRUNCATION_2, "%s must be %u-bit\n", name, n);
+		warning(
+		    WARNING_TRUNCATION_2,
+		    n == 8 && !name ? "%s must be %u-bit; use LOW() to force 8-bit\n"
+		                    : "%s must be %u-bit\n",
+		    name ? name : "Expression",
+		    n
+		);
 		return false;
 	}
 
diff --git a/src/bison.sh b/src/bison.sh
index 41a635df..3a18d076 100755
--- a/src/bison.sh
+++ b/src/bison.sh
@@ -15,14 +15,17 @@ fi
 BISON_FLAGS="-Wall -Dparse.lac=full -Dlr.type=ielr"
 
 # Set some optimization flags on versions that support them
-if [ "$BISON_MAJOR" -eq 4 ] || [ "$BISON_MAJOR" -eq 3 ] && [ "$BISON_MINOR" -ge 5 ]; then
+if [ "$BISON_MAJOR" -ge 4 ] || [ "$BISON_MAJOR" -eq 3 ] && [ "$BISON_MINOR" -ge 5 ]; then
 	BISON_FLAGS="$BISON_FLAGS -Dapi.token.raw=true"
 fi
-if [ "$BISON_MAJOR" -eq 4 ] || [ "$BISON_MAJOR" -eq 3 ] && [ "$BISON_MINOR" -ge 6 ]; then
+if [ "$BISON_MAJOR" -ge 4 ] || [ "$BISON_MAJOR" -eq 3 ] && [ "$BISON_MINOR" -ge 6 ]; then
 	BISON_FLAGS="$BISON_FLAGS -Dparse.error=detailed"
 else
 	BISON_FLAGS="$BISON_FLAGS -Dparse.error=verbose"
 fi
+if [ "$BISON_MAJOR" -ge 4 ] || [ "$BISON_MAJOR" -eq 3 ] && [ "$BISON_MINOR" -ge 7 ]; then
+	BISON_FLAGS="$BISON_FLAGS -Wcounterexamples"
+fi
 
 # Replace the arguments to this script ($@) with the ones in $BISON_FLAGS
 eval "set -- $BISON_FLAGS"
diff --git a/test/asm/ds-bad.err b/test/asm/ds-bad.err
index e573845a..dba29b84 100644
--- a/test/asm/ds-bad.err
+++ b/test/asm/ds-bad.err
@@ -1,5 +1,5 @@
 error: ds-bad.asm(3):
     Expected constant expression: 'unknown' is not constant at assembly time
 warning: ds-bad.asm(4): [-Wtruncation]
-    Expression must be 8-bit
+    Expression must be 8-bit; use LOW() to force 8-bit
 error: Assembly aborted (1 error)!
diff --git a/test/asm/invalid-instructions.asm b/test/asm/invalid-instructions.asm
index 3ed9580f..eedf4a4c 100644
--- a/test/asm/invalid-instructions.asm
+++ b/test/asm/invalid-instructions.asm
@@ -6,3 +6,16 @@ SECTION "invalid", ROM0[$10000]
 	ld b, [$4000]
 	bit 8, a
 	rst $40
+
+	ld bc, bc
+	ld de, hl
+	ld hl, de
+
+	ld hl, sp ; no offset!
+	; ld sp, hl is valid
+
+	ld sp, bc
+	ld bc, sp
+
+	ld af, bc
+	ld bc, af
diff --git a/test/asm/invalid-instructions.err b/test/asm/invalid-instructions.err
index 7a903a7d..efc6232a 100644
--- a/test/asm/invalid-instructions.err
+++ b/test/asm/invalid-instructions.err
@@ -16,4 +16,20 @@ error: invalid-instructions.asm(7):
     Invalid bit index 8 for BIT
 error: invalid-instructions.asm(8):
     Invalid address $40 for RST
-error: Assembly aborted (8 errors)!
+error: invalid-instructions.asm(10):
+    LD BC, BC is not a valid instruction; use LD B, B and LD C, C
+error: invalid-instructions.asm(11):
+    LD DE, HL is not a valid instruction; use LD D, H and LD E, L
+error: invalid-instructions.asm(12):
+    LD HL, DE is not a valid instruction; use LD H, D and LD L, E
+error: invalid-instructions.asm(14):
+    LD HL, SP is not a valid instruction; use LD HL, SP + 0
+error: invalid-instructions.asm(17):
+    LD SP, BC is not a valid instruction
+error: invalid-instructions.asm(18):
+    syntax error, unexpected sp
+error: invalid-instructions.asm(20):
+    syntax error, unexpected af
+error: invalid-instructions.asm(21):
+    syntax error, unexpected af
+error: Assembly aborted (16 errors)!
diff --git a/test/asm/warn-truncation.err b/test/asm/warn-truncation.err
index 37e4c791..b3a8a36d 100644
--- a/test/asm/warn-truncation.err
+++ b/test/asm/warn-truncation.err
@@ -1,64 +1,64 @@
 warning: warn-truncation.asm(35) -> warn-truncation.asm::try(23): [-Wtruncation]
-    Expression must be 8-bit
+    Expression must be 8-bit; use LOW() to force 8-bit
 warning: warn-truncation.asm(35) -> warn-truncation.asm::try(24): [-Wtruncation]
-    Expression must be 8-bit
+    Expression must be 8-bit; use LOW() to force 8-bit
 warning: warn-truncation.asm(35) -> warn-truncation.asm::try(25): [-Wtruncation]
     Expression must be 16-bit
 warning: warn-truncation.asm(35) -> warn-truncation.asm::try(26): [-Wtruncation]
     Expression must be 16-bit
 warning: warn-truncation.asm(35) -> warn-truncation.asm::try(28): [-Wtruncation]
-    Expression must be 8-bit
+    Expression must be 8-bit; use LOW() to force 8-bit
 warning: warn-truncation.asm(35) -> warn-truncation.asm::try(29): [-Wtruncation]
     Expression must be 16-bit
 warning: warn-truncation.asm(35) -> warn-truncation.asm::try(30): [-Wtruncation]
-    Expression must be 8-bit
+    Expression must be 8-bit; use LOW() to force 8-bit
 warning: warn-truncation.asm(35) -> warn-truncation.asm::try(31): [-Wtruncation]
     Expression must be 16-bit
 warning: warn-truncation.asm(37) -> warn-truncation.asm::try(23): [-Wtruncation]
-    Expression must be 8-bit
+    Expression must be 8-bit; use LOW() to force 8-bit
 warning: warn-truncation.asm(37) -> warn-truncation.asm::try(24): [-Wtruncation]
-    Expression must be 8-bit
+    Expression must be 8-bit; use LOW() to force 8-bit
 warning: warn-truncation.asm(37) -> warn-truncation.asm::try(25): [-Wtruncation]
     Expression must be 16-bit
 warning: warn-truncation.asm(37) -> warn-truncation.asm::try(26): [-Wtruncation]
     Expression must be 16-bit
 warning: warn-truncation.asm(38) -> warn-truncation.asm::try(23): [-Wtruncation]
-    Expression must be 8-bit
+    Expression must be 8-bit; use LOW() to force 8-bit
 warning: warn-truncation.asm(38) -> warn-truncation.asm::try(24): [-Wtruncation]
-    Expression must be 8-bit
+    Expression must be 8-bit; use LOW() to force 8-bit
 warning: warn-truncation.asm(38) -> warn-truncation.asm::try(25): [-Wtruncation]
     Expression must be 16-bit
 warning: warn-truncation.asm(38) -> warn-truncation.asm::try(26): [-Wtruncation]
     Expression must be 16-bit
 warning: warn-truncation.asm(38) -> warn-truncation.asm::try(28): [-Wtruncation]
-    Expression must be 8-bit
+    Expression must be 8-bit; use LOW() to force 8-bit
 warning: warn-truncation.asm(38) -> warn-truncation.asm::try(29): [-Wtruncation]
     Expression must be 16-bit
 warning: warn-truncation.asm(38) -> warn-truncation.asm::try(30): [-Wtruncation]
-    Expression must be 8-bit
+    Expression must be 8-bit; use LOW() to force 8-bit
 warning: warn-truncation.asm(38) -> warn-truncation.asm::try(31): [-Wtruncation]
     Expression must be 16-bit
 error: warn-truncation.asm(39) -> warn-truncation.asm::try(23): [-Werror=truncation]
-    Expression must be 8-bit
+    Expression must be 8-bit; use LOW() to force 8-bit
 error: warn-truncation.asm(39) -> warn-truncation.asm::try(24): [-Werror=truncation]
-    Expression must be 8-bit
+    Expression must be 8-bit; use LOW() to force 8-bit
 error: warn-truncation.asm(39) -> warn-truncation.asm::try(25): [-Werror=truncation]
     Expression must be 16-bit
 error: warn-truncation.asm(39) -> warn-truncation.asm::try(26): [-Werror=truncation]
     Expression must be 16-bit
 error: warn-truncation.asm(40) -> warn-truncation.asm::try(23): [-Werror=truncation]
-    Expression must be 8-bit
+    Expression must be 8-bit; use LOW() to force 8-bit
 error: warn-truncation.asm(40) -> warn-truncation.asm::try(24): [-Werror=truncation]
-    Expression must be 8-bit
+    Expression must be 8-bit; use LOW() to force 8-bit
 error: warn-truncation.asm(40) -> warn-truncation.asm::try(25): [-Werror=truncation]
     Expression must be 16-bit
 error: warn-truncation.asm(40) -> warn-truncation.asm::try(26): [-Werror=truncation]
     Expression must be 16-bit
 error: warn-truncation.asm(40) -> warn-truncation.asm::try(28): [-Werror=truncation]
-    Expression must be 8-bit
+    Expression must be 8-bit; use LOW() to force 8-bit
 error: warn-truncation.asm(40) -> warn-truncation.asm::try(29): [-Werror=truncation]
     Expression must be 16-bit
 error: warn-truncation.asm(40) -> warn-truncation.asm::try(30): [-Werror=truncation]
-    Expression must be 8-bit
+    Expression must be 8-bit; use LOW() to force 8-bit
 error: warn-truncation.asm(40) -> warn-truncation.asm::try(31): [-Werror=truncation]
     Expression must be 16-bit