diff --git a/block/dmg.c b/block/dmg.c
index c1066df13a..3141cb5b88 100644
--- a/block/dmg.c
+++ b/block/dmg.c
@@ -51,9 +51,16 @@ typedef struct BDRVDMGState {
 
 static int dmg_probe(const uint8_t *buf, int buf_size, const char *filename)
 {
-    int len=strlen(filename);
-    if(len>4 && !strcmp(filename+len-4,".dmg"))
-	return 2;
+    int len;
+
+    if (!filename) {
+        return 0;
+    }
+
+    len = strlen(filename);
+    if (len > 4 && !strcmp(filename + len - 4, ".dmg")) {
+        return 2;
+    }
     return 0;
 }