diff --git a/pymbolic/mapper/differentiator.py b/pymbolic/mapper/differentiator.py
index b59c9b1b554fffa80477294faba5b40361cadeab..e0e0aea042d9bbb741ec829cb50fbc1a9b4e8a41 100644
--- a/pymbolic/mapper/differentiator.py
+++ b/pymbolic/mapper/differentiator.py
@@ -59,6 +59,7 @@ def map_math_functions_by_name(i, func, pars):
     elif f is math.expm1 and len(pars) == 1:
         return make_f("exp")(*pars)
     elif f is math.fabs and len(pars) == 1:
+        from pymbolic.functions import sgn
         return sgn(*pars)
     else:
         raise RuntimeError("unrecognized function, cannot differentiate")