diff --git a/Python/pystrtod.c b/Python/pystrtod.c
index 5fa51eb..0dea0a6 100644
--- a/Python/pystrtod.c
+++ b/Python/pystrtod.c
@@ -1001,9 +1001,17 @@
         else {
             /* shouldn't get here: Gay's code should always return
                something starting with a digit, an 'I',  or 'N' */
-            strncpy(p, "ERR", 3);
-            p += 3;
-            /* assert(0); */
+            printf("Unexpected failure in format_float_short. "
+                   "Arguments: d = %.17g, format_code = %d, "
+                   "mode = %d, precision = %ld\n",
+                   d, format_code, mode, precision);
+            printf("digits == %.100s\n", digits);
+            PyErr_Format(PyExc_RuntimeError,
+                         "Unexpected failure in format_float_short. "
+                         "Arguments: d = %.17g, format_code = %d, "
+                         "mode = %d, precision = %ld\n",
+                         d, format_code, mode, precision);
+            return NULL;
         }
         goto exit;
     }