diff --git a/include/vm.h b/include/vm.h
index 71abbe6..b1bca0c 100644
--- a/include/vm.h
+++ b/include/vm.h
@@ -113,6 +113,8 @@ bool library_function_exists(int libno, int fno);
 void init_libraries(void);
 void exit_libraries(void);
 
+struct string *string_format(struct string *fmt, union vm_value arg, enum ain_data_type type);
+
 void vm_stack_trace(void);
 _Noreturn void _vm_error(const char *fmt, ...);
 _Noreturn void vm_exit(int code);
diff --git a/src/format.c b/src/format.c
index 686a8c0..052dc6c 100644
--- a/src/format.c
+++ b/src/format.c
@@ -17,6 +17,7 @@
 #include <stdbool.h>
 #include <string.h>
 #include "system4.h"
+#include "system4/ain.h"
 #include "system4/string.h"
 #include "vm.h"
 #include "vm/heap.h"
@@ -53,35 +54,43 @@ static int read_number(const char **_fmt)
 	return n;
 }
 
-static void parse_fmt_spec(const char **_fmt, struct fmt_spec *spec)
+static bool parse_fmt_spec(const char **_fmt, struct fmt_spec *spec, enum ain_data_type target)
 {
-	const char *fmt = *_fmt;
+	bool r = false;
+	const char *fmt = (*_fmt) + 1;
 	memset(spec, 0, sizeof(struct fmt_spec));
 	spec->precision = 6;
 	while (*fmt) {
 		switch (*fmt) {
 		case 'd':
 			spec->type = FMT_INT;
+			r = target == AIN_INT;
 			goto end;
 		case 'D':
 			spec->type = FMT_INT;
 			spec->zenkaku = true;
+			r = target == AIN_INT;
 			goto end;
 		case 'f':
 			spec->type = FMT_FLOAT;
+			r = target == AIN_FLOAT;
 			goto end;
 		case 'F':
 			spec->type = FMT_FLOAT;
 			spec->zenkaku = true;
+			r = target == AIN_FLOAT;
 			goto end;
 		case 's':
 			spec->type = FMT_STRING;
+			r = target == AIN_STRING;
 			goto end;
 		case 'c':
 			spec->type = FMT_CHAR;
+			r = target == AIN_INT;
 			goto end;
 		case 'b':
 			spec->type = FMT_BOOL;
+			r = target == AIN_INT;
 			goto end;
 		case '0':
 			spec->zero_pad = true;
@@ -109,7 +118,9 @@ static void parse_fmt_spec(const char **_fmt, struct fmt_spec *spec)
 warn:
 	WARNING("Invalid format specifier: %s", *_fmt);
 end:
-	*_fmt = ++fmt;
+	if (r)
+		*_fmt = ++fmt;
+	return r;
 }
 
 static void append_fmt(struct string **s, struct fmt_spec *spec, union vm_value arg)
@@ -128,7 +139,7 @@ static void append_fmt(struct string **s, struct fmt_spec *spec, union vm_value
 		string_append_cstr(s, buf, len);
 		break;
 	case FMT_STRING:
-		string_append(s, heap[arg.i].s);
+		string_append(s, heap_get_string(arg.i));
 		heap_unref(arg.i);
 		break;
 	case FMT_CHAR:
@@ -143,15 +154,17 @@ static void append_fmt(struct string **s, struct fmt_spec *spec, union vm_value
 	}
 }
 
-struct string *string_format(struct string *fmt, union vm_value arg)
+struct string *string_format(struct string *fmt, union vm_value arg, enum ain_data_type type)
 {
 	struct string *out = string_ref(&EMPTY_STRING);
 	for (const char *s = fmt->text; *s; s++) {
-		if (*s == '%') {
-			struct fmt_spec spec;
-			string_append_cstr(&out, fmt->text, s - fmt->text);
-			s++;
-			parse_fmt_spec(&s, &spec);
+		if (*s != '%')
+			continue;
+
+		size_t size = s - fmt->text;
+		struct fmt_spec spec;
+		if (parse_fmt_spec(&s, &spec, type)) {
+			string_append_cstr(&out, fmt->text, size);
 			append_fmt(&out, &spec, arg);
 			string_append_cstr(&out, s, strlen(s));
 			break;
diff --git a/src/parts/parts.c b/src/parts/parts.c
index 3e7e5eb..de9b7a7 100644
--- a/src/parts/parts.c
+++ b/src/parts/parts.c
@@ -960,7 +960,7 @@ bool PE_SetLoopCG_by_index(int parts_no, int cg_no, int nr_frames, int frame_tim
 static struct cg *load_loop_cg_by_name(int no, void *data)
 {
 	int unused_no;
-	struct string *cg_name = string_format((struct string*)data, (union vm_value){.i=no});
+	struct string *cg_name = string_format((struct string*)data, (union vm_value){.i=no}, AIN_INT);
 	struct cg *cg = asset_cg_load_by_name(cg_name->text, &unused_no);
 	free_string(cg_name);
 	return cg;
diff --git a/src/vm.c b/src/vm.c
index 94e2619..78131ef 100644
--- a/src/vm.c
+++ b/src/vm.c
@@ -1499,11 +1499,11 @@ static enum opcode execute_instruction(enum opcode opcode)
 		break;
 	}
 	case S_MOD: {
-		stack_pop(); // ???
+		int type = stack_pop().i;
 		union vm_value val = stack_pop();
 		int fmt = stack_pop().i;
 		int dst = heap_alloc_slot(VM_STRING);
-		heap[dst].s = string_format(heap[fmt].s, val);
+		heap[dst].s = string_format(heap[fmt].s, val, type + 8);
 		heap_unref(fmt);
 		stack_push(dst);
 		break;
diff --git a/subprojects/libsys4 b/subprojects/libsys4
index 863bd9d..95ce167 160000
--- a/subprojects/libsys4
+++ b/subprojects/libsys4
@@ -1 +1 @@
-Subproject commit 863bd9d332d74fc843b0b5f8b26f774db1ab8d7b
+Subproject commit 95ce167a93fcc3348431583d2f7f0b3637b12f79
diff --git a/test/Source/strings.jaf b/test/Source/strings.jaf
index b7aef4e..98c2904 100755
--- a/test/Source/strings.jaf
+++ b/test/Source/strings.jaf
@@ -73,6 +73,7 @@ void test_strings(void)
 	test_string("abc%4Ddef % 42", "abc%4Ddef" % 42, "abc@@‚S‚Qdef");
 	test_string("abc%04Ddef % 42", "abc%04Ddef" % 42, "abc‚O‚O‚S‚Qdef");
 	test_string("\"%d %d\" % 42 % 43", "%d %d" % 42 % 43, "42 43");
+	test_string("%s %d % 42", "%s %d" % 42, "%s 42");
 	// float
 	test_string("abc%fdef % 12.3", "abc%fdef" % 12.3, "abc12.300000def");
 	test_string("abc%.2fdef % 12.3", "abc%.2fdef" % 12.3, "abc12.30def");