diff --git a/src/cairo-pdf-surface.c b/src/cairo-pdf-surface.c
index 9b2b93252..dcecfb23a 100644
--- a/src/cairo-pdf-surface.c
+++ b/src/cairo-pdf-surface.c
@@ -779,7 +779,7 @@ cairo_pdf_get_versions (cairo_pdf_version_t const	**versions,
 const char *
 cairo_pdf_version_to_string (cairo_pdf_version_t version)
 {
-    if (version >= CAIRO_PDF_VERSION_LAST)
+    if (version < 0 || version >= CAIRO_PDF_VERSION_LAST)
 	return NULL;
 
     return _cairo_pdf_version_strings[version];