Fix sympy error

cirq-core/cirq/study/resolver.py

@@ -112,6 +112,7 @@ def value_of(
         Raises:
             RecursionError: If the ParamResolver detects a loop in recursive
                 resolution.
+            ValueError: If the resulting value cannot be interpreted.
         """
 
         # Input is a pass through type, no resolution needed: return early
@@ -179,7 +180,13 @@ def value_of(
         if not recursive:
             # Resolves one step at a time. For example:
             # a.subs({a: b, b: c}) == b
-            v = value.subs(self.param_dict, simultaneous=True)
+            try:
+                v = value.subs(self.param_dict, simultaneous=True)
+            except sympy.SympifyError as e:  # coverage: ignore
+                # Lines will be covered in sympy 1.12+
+                raise ValueError(
+                    f'Could not resolve parameter {value}, underlying error {e}'
+                )  # coverage: ignore
             if v.free_symbols:
                 return v
             elif sympy.im(v):

cirq-core/cirq/study/resolver_test.py

@@ -244,9 +244,11 @@ def _resolved_value_(self):
     c = sympy.Symbol('c')
     r = cirq.ParamResolver({a: foo, b: bar, c: baz})
     assert r.value_of(a) is foo
-    assert r.value_of(b) is b
     assert r.value_of(c) == 'Baz'
 
+    with pytest.raises(ValueError, match='Could not resolve parameter b'):
+        _ = r.value_of(b)
+
 
 def test_compose():
     """Tests that cirq.resolve_parameters on a ParamResolver composes."""