diff --git a/test/asm/label-diff.asm b/test/asm/label-diff.asm
index 493def1e..fa81e945 100644
--- a/test/asm/label-diff.asm
+++ b/test/asm/label-diff.asm
@@ -36,6 +36,8 @@ POPS ; Ensure we are in neither section
 	print_diff Constant, Unknown
 	; Diffing a floating label and a ref cannot work
 	print_diff Known, Unknown
+	; Diffing two refs cannot work
+	print_diff Unknown, Unknown2
 
 	; Now let's fiddle with PC
 SECTION "fixed PC", ROM0[420]
diff --git a/test/asm/label-diff.err b/test/asm/label-diff.err
index f6a2d763..f098b2bc 100644
--- a/test/asm/label-diff.err
+++ b/test/asm/label-diff.err
@@ -10,24 +10,28 @@ ERROR: label-diff.asm(38) -> label-diff.asm::print_diff(20):
     Expected constant expression: 'Known' is not constant at assembly time
 ERROR: label-diff.asm(38) -> label-diff.asm::print_diff(22):
     Expected constant expression: 'Unknown' is not constant at assembly time
-ERROR: label-diff.asm(45) -> label-diff.asm::print_diff(20):
-    Expected constant expression: 'Known' is not constant at assembly time
-ERROR: label-diff.asm(45) -> label-diff.asm::print_diff(22):
-    Expected constant expression: 'Known' is not constant at assembly time
+ERROR: label-diff.asm(40) -> label-diff.asm::print_diff(20):
+    Expected constant expression: 'Unknown' is not constant at assembly time
+ERROR: label-diff.asm(40) -> label-diff.asm::print_diff(22):
+    Expected constant expression: 'Unknown2' is not constant at assembly time
 ERROR: label-diff.asm(47) -> label-diff.asm::print_diff(20):
-    Expected constant expression: 'Unknown' is not constant at assembly time
-ERROR: label-diff.asm(47) -> label-diff.asm::print_diff(22):
-    Expected constant expression: 'Unknown' is not constant at assembly time
-ERROR: label-diff.asm(56) -> label-diff.asm::print_diff(20):
-    Expected constant expression: PC is not constant at assembly time
-ERROR: label-diff.asm(56) -> label-diff.asm::print_diff(22):
-    Expected constant expression: PC is not constant at assembly time
-ERROR: label-diff.asm(58) -> label-diff.asm::print_diff(20):
     Expected constant expression: 'Known' is not constant at assembly time
+ERROR: label-diff.asm(47) -> label-diff.asm::print_diff(22):
+    Expected constant expression: 'Known' is not constant at assembly time
+ERROR: label-diff.asm(49) -> label-diff.asm::print_diff(20):
+    Expected constant expression: 'Unknown' is not constant at assembly time
+ERROR: label-diff.asm(49) -> label-diff.asm::print_diff(22):
+    Expected constant expression: 'Unknown' is not constant at assembly time
+ERROR: label-diff.asm(58) -> label-diff.asm::print_diff(20):
+    Expected constant expression: PC is not constant at assembly time
 ERROR: label-diff.asm(58) -> label-diff.asm::print_diff(22):
     Expected constant expression: PC is not constant at assembly time
 ERROR: label-diff.asm(60) -> label-diff.asm::print_diff(20):
-    Expected constant expression: 'Unknown' is not constant at assembly time
+    Expected constant expression: 'Known' is not constant at assembly time
 ERROR: label-diff.asm(60) -> label-diff.asm::print_diff(22):
     Expected constant expression: PC is not constant at assembly time
-error: Assembly aborted (16 errors)!
+ERROR: label-diff.asm(62) -> label-diff.asm::print_diff(20):
+    Expected constant expression: 'Unknown' is not constant at assembly time
+ERROR: label-diff.asm(62) -> label-diff.asm::print_diff(22):
+    Expected constant expression: PC is not constant at assembly time
+error: Assembly aborted (18 errors)!
diff --git a/test/asm/label-diff.out b/test/asm/label-diff.out
index 2ec5e077..bb02119d 100644
--- a/test/asm/label-diff.out
+++ b/test/asm/label-diff.out
@@ -8,6 +8,8 @@ $0
 $0
 $0
 $0
+$0
+$0
 $FFFFFE86
 $17A
 $0