diff --git a/src/diag.c b/src/diag.c
index d97be9b96b..8512063a2e 100644
--- a/src/diag.c
+++ b/src/diag.c
@@ -9,7 +9,9 @@
 
 void avifDiagnosticsClearError(avifDiagnostics * diag)
 {
-    *diag->error = '\0';
+    if (diag) {
+        *diag->error = '\0';
+    }
 }
 
 #ifdef __clang__
@@ -18,7 +20,6 @@ __attribute__((__format__(__printf__, 2, 3)))
 void avifDiagnosticsPrintf(avifDiagnostics * diag, const char * format, ...)
 {
     if (!diag) {
-        // It is possible this is NULL (e.g. calls to avifFileTypeIsCompatible())
         return;
     }
     if (*diag->error) {