diff --git a/py/strtonum.c b/py/strtonum.c
index 02c1506683..48a746603f 100644
--- a/py/strtonum.c
+++ b/py/strtonum.c
@@ -60,6 +60,7 @@ long strtonum(const char *restrict s, int base) {
         p--;
     }
 
+    errno = 0;
     found = strtol(p, &num, base);
     if (errno) {
         goto value_error;