diff --git a/src/asm/actions.cpp b/src/asm/actions.cpp
index 7ab356e5..7d011b16 100644
--- a/src/asm/actions.cpp
+++ b/src/asm/actions.cpp
@@ -555,7 +555,7 @@ std::string act_StringFormat(
 	std::string str;
 	size_t argIndex = 0;
 
-	for (size_t i = 0; spec[i] != '\0';) {
+	for (size_t i = 0; i < spec.length();) {
 		if (int c = spec[i]; c != '%') {
 			str += c;
 			++i;
diff --git a/test/asm/strfmt.asm b/test/asm/strfmt.asm
index 80a16f29..1ba29982 100644
--- a/test/asm/strfmt.asm
+++ b/test/asm/strfmt.asm
@@ -26,3 +26,7 @@ PRINTLN STRFMT("%d eol %", 1)
 PRINTLN STRFMT("invalid %w spec", 42)
 
 PRINTLN STRFMT("one=%d two=%d three=%d", 1)
+
+DEF NUL EQUS STRFMT("%s \0 %s", "goodbye", "world")
+ASSERT #NUL === "goodbye \0 world"
+PRINTLN #NUL
diff --git a/test/asm/strfmt.out b/test/asm/strfmt.out
index 2f3ec1cc..43f452d5 100644
Binary files a/test/asm/strfmt.out and b/test/asm/strfmt.out differ