diff --git a/waveflag.c b/waveflag.c
index 605b9b1a7..3c234e0af 100644
--- a/waveflag.c
+++ b/waveflag.c
@@ -409,7 +409,12 @@ wave_flag (const char *filename, const char *out_prefix)
 	*out = '\0';
 	strcat (out, out_prefix);
         // diff from upstream. we call this a bit differently, filename might not be in cwd.
-	strcat (out, basename(filename));
+
+        // basename wants a non-const argument.  The problem here is paths that end in a
+        // slash, POSIX basename removes them while GNU just returns a pointer to that
+        // slash.  Since this is supposed to be a filename such input is illegal for us.
+        // We're already not checking for overflow of the output buffer anyway...
+	strcat (out, basename((char *) filename));
 
 	cairo_surface_write_to_png (cairo_get_target (cr), out);
 	cairo_destroy (cr);