diff --git a/apps/zasm/const.asm b/apps/zasm/const.asm
new file mode 100644
index 0000000..3972478
--- /dev/null
+++ b/apps/zasm/const.asm
@@ -0,0 +1,8 @@
+; *** Errors ***
+; Unknown instruction or directive
+.equ	ERR_UNKNOWN		0x01
+
+; Bad argument: Doesn't match any constant argspec or, if an expression,
+; contains references to undefined symbols.
+.equ	ERR_BAD_ARG		0x02
+
diff --git a/apps/zasm/glue.asm b/apps/zasm/glue.asm
index 47c1492..8d03714 100644
--- a/apps/zasm/glue.asm
+++ b/apps/zasm/glue.asm
@@ -45,6 +45,7 @@
 
 jp	zasmMain
 
+#include "zasm/const.asm"
 #include "zasm/util.asm"
 .equ	IO_RAMSTART	USER_RAMSTART
 #include "zasm/io.asm"
diff --git a/apps/zasm/instr.asm b/apps/zasm/instr.asm
index b7a94e2..cefff68 100644
--- a/apps/zasm/instr.asm
+++ b/apps/zasm/instr.asm
@@ -766,12 +766,13 @@ processArg:
 	cp	a		; ensure Z is set
 	ret
 .error:
+	ld	a, ERR_BAD_ARG
 	call	unsetZ
 	ret
 
 ; Parse instruction specified in A (I_* const) with args in I/O and write
 ; resulting opcode(s) in I/O.
-; Sets Z on success.
+; Sets Z on success. On error, A contains an error code (ERR_*)
 parseInstruction:
 	push	bc
 	push	hl
@@ -786,14 +787,14 @@ parseInstruction:
 	jr	nz, .nomorearg
 	ld	de, curArg1
 	call	processArg
-	jr	nz, .error
+	jr	nz, .error	; A is set to error
 	call	readComma
 	jr	nz, .nomorearg
 	call	readWord
 	jr	nz, .error
 	ld	de, curArg2
 	call	processArg
-	jr	nz, .error
+	jr	nz, .error	; A is set to error
 .nomorearg:
 	; Parsing done, no error, let's move forward to instr row matching!
 	ld	de, instrTBl
@@ -824,6 +825,7 @@ parseInstruction:
 	cp	a	; ensure Z
 	jr	.end
 .error:
+	; A is set to error already
 	call	unsetZ
 .end:
 	pop	de
diff --git a/apps/zasm/main.asm b/apps/zasm/main.asm
index 8ee4477..23e15fe 100644
--- a/apps/zasm/main.asm
+++ b/apps/zasm/main.asm
@@ -15,10 +15,6 @@
 .equ	ZASM_ORG		ZASM_CTX_PC+2
 .equ	ZASM_RAMEND		ZASM_ORG+2
 
-; *** Errors ***
-; Unknown instruction or directive
-.equ	ERR_UNKWN		0x01
-
 ; Read file through blockdev ID in H and outputs its upcodes through blockdev
 ; ID in L.
 zasmMain:
@@ -115,7 +111,7 @@ parseLine:
 	cp	TOK_EOF
 	ret	z		; We're finished, no error.
 	; Bad token
-	ld	a, ERR_UNKWN
+	ld	a, ERR_UNKNOWN
 	jp	unsetZ		; return with Z unset
 
 _parseInstr:
diff --git a/tools/emul/zasm/zasm.bin b/tools/emul/zasm/zasm.bin
index 8194765..dcd66d3 100644
Binary files a/tools/emul/zasm/zasm.bin and b/tools/emul/zasm/zasm.bin differ
diff --git a/tools/tests/zasm/errtests.sh b/tools/tests/zasm/errtests.sh
index a9e9de9..7bf15b2 100755
--- a/tools/tests/zasm/errtests.sh
+++ b/tools/tests/zasm/errtests.sh
@@ -17,4 +17,4 @@ chkerr() {
 }
 
 chkerr "foo" 1
-
+chkerr "ld a, foo" 2
diff --git a/tools/tests/zasm/test7.asm b/tools/tests/zasm/test7.asm
index 3894ff3..041f81b 100644
--- a/tools/tests/zasm/test7.asm
+++ b/tools/tests/zasm/test7.asm
@@ -21,6 +21,7 @@
 .equ    fsSeek         0x30
 .equ    fsTell         0x33
 
+#include "zasm/const.asm"
 #include "zasm/util.asm"
 .equ	IO_RAMSTART	USER_RAMSTART
 #include "zasm/io.asm"