diff --git a/src/json.c b/src/json.c
index 928825e034..9c03d09ca6 100644
--- a/src/json.c
+++ b/src/json.c
@@ -225,8 +225,25 @@ json_has_suffix (const char *string, const char *suffix)
 static Lisp_Object
 json_make_string (const char *data, ptrdiff_t size)
 {
-  return code_convert_string (make_specified_string (data, -1, size, false),
-                              Qutf_8_unix, Qt, false, true, true);
+  char* ptr = (char*)data + size;
+  bool ascii_only = true;
+
+  while (ptr > data) {
+    if (!ASCII_CHAR_P(*ptr))
+      {
+        ascii_only = false;
+        break;
+      }
+    ptr--;
+  }
+
+  if (ascii_only)
+    /* Wasn't sure if the last arg should better be true here, but
+       it doesn't seem to affect the behavior or correctness, so far. */
+    return make_specified_string (data, -1, size, false);
+  else
+    return code_convert_string (make_specified_string (data, -1, size, false),
+                                Qutf_8_unix, Qt, false, true, true);
 }
 
 /* Create a multibyte Lisp string from the NUL-terminated UTF-8