diff --git a/json5/lib.py b/json5/lib.py
index 7284003..d14c410 100644
--- a/json5/lib.py
+++ b/json5/lib.py
@@ -680,7 +680,7 @@ def _encode_float(self, obj: float, *, as_key: bool) -> str:
             s = float.__repr__(obj)
 
         if not allowed:
-            raise ValueError('Illegal JSON5 value: f{obj}')
+            raise ValueError(f'Illegal JSON5 value: {obj}')
         return f'"{s}"' if as_key else s
 
     def _encode_str(self, obj: str, *, as_key: bool) -> str:
diff --git a/tests/lib_test.py b/tests/lib_test.py
index 72b85da..d48a82a 100644
--- a/tests/lib_test.py
+++ b/tests/lib_test.py
@@ -583,15 +583,10 @@ def test_numbers(self):
         self.check(float('-inf'), '-Infinity')
         self.check(float('nan'), 'NaN')
 
-        self.assertRaises(
-            ValueError, json5.dumps, float('inf'), allow_nan=False
-        )
-        self.assertRaises(
-            ValueError, json5.dumps, float('-inf'), allow_nan=False
-        )
-        self.assertRaises(
-            ValueError, json5.dumps, float('nan'), allow_nan=False
-        )
+        for v in (float('inf'), float('-inf'), float('nan')):
+            with self.assertRaises(ValueError) as ctx:
+                json5.dumps(v, allow_nan=False)
+            self.assertIn(str(v), str(ctx.exception))
 
     def test_null(self):
         self.check(None, 'null')