diff --git a/src/libzzuf/debug.c b/src/libzzuf/debug.c
index c57e2e1..a674f3c 100644
--- a/src/libzzuf/debug.c
+++ b/src/libzzuf/debug.c
@@ -331,14 +331,14 @@ static void mydebug(char const *format, va_list args)
     errno = saved_errno;
 }
 
-void zzuf_debug_str(char *str, uint8_t const *buffer,
-                    unsigned len, unsigned maxlen)
+void zzuf_debug_str(char *str, uint8_t const *buffer, int len, int maxlen)
 {
     /* Open the double quotes */
-    *str++ = '"';
+    if (len >= 0)
+        *str++ = '"';
 
     /* Print as many escaped characters as possible */
-    for (unsigned i = 0; i < len; ++i)
+    for (int i = 0; i < len; ++i)
     {
         if (len > maxlen && i == maxlen / 2)
         {
@@ -372,7 +372,8 @@ void zzuf_debug_str(char *str, uint8_t const *buffer,
     }
 
     /* Close the double quotes */
-    *str++ = '"';
+    if (len >= 0)
+        *str++ = '"';
     *str++ = '\0';
 }
 
diff --git a/src/libzzuf/debug.h b/src/libzzuf/debug.h
index 43b6d54..d628f0e 100644
--- a/src/libzzuf/debug.h
+++ b/src/libzzuf/debug.h
@@ -17,7 +17,7 @@
 extern void zzuf_debug(char const *format, ...) ATTRIBUTE_PRINTF(1,2);
 extern void zzuf_debug2(char const *format, ...) ATTRIBUTE_PRINTF(1,2);
 extern void zzuf_debug_str(char *str, uint8_t const *buffer,
-                           unsigned len, unsigned maxlen);
+                           int len, int maxlen);
 
 #ifdef LIBZZUF
 #   define debug zzuf_debug