diff --git a/mercurial/templatefilters.py b/mercurial/templatefilters.py
--- a/mercurial/templatefilters.py
+++ b/mercurial/templatefilters.py
@@ -219,7 +219,7 @@ def json(obj):
         raise TypeError('cannot encode type %s' % obj.__class__.__name__)
 
 def _uescape(c):
-    if ord(c) < 0x80:
+    if 0x20 <= ord(c) < 0x80:
         return c
     else:
         return '\\u%04x' % ord(c)
diff --git a/tests/test-template-engine.t b/tests/test-template-engine.t
--- a/tests/test-template-engine.t
+++ b/tests/test-template-engine.t
@@ -44,4 +44,17 @@
   0 97e5f848f0936960273bbf75be6388cd0350a32b -1 0000000000000000000000000000000000000000
   -1 0000000000000000000000000000000000000000 -1 0000000000000000000000000000000000000000
 
+Fuzzing the unicode escaper to ensure it produces valid data
+
+#if hypothesis
+
+  >>> from hypothesishelpers import *
+  >>> import mercurial.templatefilters as tf
+  >>> import json
+  >>> @check(st.text().map(lambda s: s.encode('utf-8')))
+  ... def testtfescapeproducesvalidjson(text):
+  ...     json.loads('"' + tf.jsonescape(text) + '"')
+
+#endif
+
   $ cd ..