diff --git a/src/json.c b/src/json.c
index 56bf734..b95e7df 100644
--- a/src/json.c
+++ b/src/json.c
@@ -219,7 +219,11 @@
                 /* Basic Multilingual Plane character \uXXXX */
                 offset++;
                 for (value = i = 0; i < 4; i++) {
-                    if (isdigit(in[offset + i])) {
+                    if (!in[offset + i]) {
+                        LOGVAL(jsonctx->ctx, LY_VLOG_LINE, &jsonctx->line, LYVE_SYNTAX,
+                            "Invalid basic multilingual plane character \"%s\".", &in[slash]);
+                        goto error;
+                    } else if (isdigit(in[offset + i])) {
                         u = (in[offset + i] - '0');
                     } else if (in[offset + i] > 'F') {
                         u = 10 + (in[offset + i] - 'a');