diff --git a/mpmath/libmp/libmpf.py b/mpmath/libmp/libmpf.py
index 18399b15..366909d2 100644
--- a/mpmath/libmp/libmpf.py
+++ b/mpmath/libmp/libmpf.py
@@ -1307,9 +1307,11 @@ def str_to_man_exp(x, base=10):
     parts = x.split(sep)
     if len(parts) == 1:
         exp = 0
-    else: # == 2
+    elif len(parts) == 2:
         x = parts[0]
         exp = int(parts[1])
+    else:
+        raise ValueError("couldn't convert a str to mpf")
     # Look for radix point in mantissa
     parts = x.split('.')
     if len(parts) == 2:
diff --git a/mpmath/tests/test_convert.py b/mpmath/tests/test_convert.py
index 1c40af0f..bdb5ca97 100644
--- a/mpmath/tests/test_convert.py
+++ b/mpmath/tests/test_convert.py
@@ -79,6 +79,7 @@ def test_to_str():
     x = mpf('1234.567891')._mpf_
     pytest.raises(ValueError, lambda: to_str(x, 6, binary_exp=True))
     pytest.raises(ValueError, lambda: to_str(x, 6, rounding='Y'))
+    pytest.raises(ValueError, lambda: to_str('1e400e2', 6))
     assert to_str(x, 5, rounding='n') == '1234.6'
     assert to_str(x, 5, rounding='d') == '1234.5'
     assert to_str(x, 5, rounding='u') == '1234.6'