diff --git a/programs/x509/cert_write.c b/programs/x509/cert_write.c
index 40b1871..5e0d608 100644
--- a/programs/x509/cert_write.c
+++ b/programs/x509/cert_write.c
@@ -583,6 +583,9 @@
 
                 if ((subtype_value = strchr(q, ':')) != NULL) {
                     *subtype_value++ = '\0';
+                } else {
+                    mbedtls_printf("Invalid argument for option SAN: Entry should be seperated by a colon\n");
+                    goto usage;
                 }
                 if (strcmp(q, "RFC822") == 0) {
                     cur->node.type = MBEDTLS_X509_SAN_RFC822_NAME;