diff --git a/src/asm/lexer.cpp b/src/asm/lexer.cpp
index a485b19a..ef65e7e7 100644
--- a/src/asm/lexer.cpp
+++ b/src/asm/lexer.cpp
@@ -724,7 +724,7 @@ static uint32_t readBracketedMacroArgNum(void)
 		}
 
 		if (i == sizeof(symName)) {
-			warning(WARNING_LONG_STR, "Bracketed symbol name too long\n");
+			warning(WARNING_LONG_STR, "Bracketed symbol name too long, got truncated\n");
 			i--;
 		}
 		symName[i] = '\0';
@@ -1371,10 +1371,14 @@ static char const *readInterpolation(size_t depth)
 			break;
 		} else if (c == ':' && !fmt_IsFinished(&fmt)) { // Format spec, only once
 			shiftChar();
+			if (i == sizeof(symName)) {
+				warning(WARNING_LONG_STR, "Format spec too long, got truncated\n");
+				i = sizeof(symName) - 1;
+			}
+			symName[i] = '\0';
 			for (size_t j = 0; j < i; j++)
 				fmt_UseCharacter(&fmt, symName[j]);
 			fmt_FinishCharacters(&fmt);
-			symName[i] = '\0';
 			if (!fmt_IsValid(&fmt))
 				error("Invalid format spec '%s'\n", symName);
 			i = 0; // Now that format has been set, restart at beginning of string
@@ -1386,7 +1390,7 @@ static char const *readInterpolation(size_t depth)
 	}
 
 	if (i == sizeof(symName)) {
-		warning(WARNING_LONG_STR, "Interpolated symbol name too long\n");
+		warning(WARNING_LONG_STR, "Interpolated symbol name too long, got truncated\n");
 		i--;
 	}
 	symName[i] = '\0';
diff --git a/test/asm/bracketed-macro-args.err b/test/asm/bracketed-macro-args.err
index a7dd5f2c..dc451831 100644
--- a/test/asm/bracketed-macro-args.err
+++ b/test/asm/bracketed-macro-args.err
@@ -9,7 +9,7 @@ error: bracketed-macro-args.asm(33) -> bracketed-macro-args.asm::bad(29):
 error: bracketed-macro-args.asm(33) -> bracketed-macro-args.asm::bad(30):
     Macro argument '\<2>' not defined
 warning: bracketed-macro-args.asm(39) -> bracketed-macro-args.asm::toolong(36): [-Wlong-string]
-    Bracketed symbol name too long
+    Bracketed symbol name too long, got truncated
 error: bracketed-macro-args.asm(39) -> bracketed-macro-args.asm::toolong(36):
     Bracketed symbol "abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstu" does not exist
 error: Assembly aborted (6 errors)!
diff --git a/test/asm/long-format-spec.asm b/test/asm/long-format-spec.asm
new file mode 100644
index 00000000..a88a1c4f
--- /dev/null
+++ b/test/asm/long-format-spec.asm
@@ -0,0 +1,3 @@
+DEF n EQU 42
+; the format spec is truncated to "000...009", omitting the "x", so this prints "000000042" not "00000002a"
+PRINTLN "{000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000009x:n}"
diff --git a/test/asm/long-format-spec.err b/test/asm/long-format-spec.err
new file mode 100644
index 00000000..7f3adc8a
--- /dev/null
+++ b/test/asm/long-format-spec.err
@@ -0,0 +1,5 @@
+warning: long-format-spec.asm(3): [-Wlong-string]
+    Format spec too long, got truncated
+error: long-format-spec.asm(3):
+    Invalid format spec '000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000009'
+error: Assembly aborted (1 error)!
diff --git a/test/asm/long-format-spec.out b/test/asm/long-format-spec.out
new file mode 100644
index 00000000..595d3c17
--- /dev/null
+++ b/test/asm/long-format-spec.out
@@ -0,0 +1 @@
+000000042