diff --git a/jax/_src/interpreters/partial_eval.py b/jax/_src/interpreters/partial_eval.py
index 5406aad172c4..2d27bf064fce 100644
--- a/jax/_src/interpreters/partial_eval.py
+++ b/jax/_src/interpreters/partial_eval.py
@@ -2062,11 +2062,9 @@ def process_primitive(self, primitive, tracers, params):
   def default_process_primitive(self, primitive, tracers, params):
     avals = [t.aval for t in tracers]
     out_avals, effects = primitive.abstract_eval(*avals, **params)
-    # == serve as a "not xor" here.
-    if not (isinstance(out_avals, (tuple,list)) == primitive.multiple_results):
-      raise ValueError(f"{primitive}.abstract_eval() method should return"
-                       f" a tuple or a list if {primitive}.multiple_results"
-                       " is true. Otherwise it shouldn't.")
+    if isinstance(out_avals, (tuple, list)) != primitive.multiple_results:
+      raise ValueError(f"{primitive}.abstract_eval() method should return "
+                       f"a tuple or a list iff {primitive}.multiple_results.")
     out_avals = [out_avals] if not primitive.multiple_results else out_avals
     source_info = source_info_util.current()
     out_tracers = [DynamicJaxprTracer(self, a, source_info) for a in out_avals]