diff --git a/bigint/src/biguint.rs b/bigint/src/biguint.rs
index fe6303e..00877c8 100644
--- a/bigint/src/biguint.rs
+++ b/bigint/src/biguint.rs
@@ -242,7 +242,9 @@ impl Num for BigUint {
                 v.push(d);
             } else {
                 // create ParseIntError::InvalidDigit
-                let e = u64::from_str_radix(&s[v.len()..], radix).unwrap_err();
+                // Include the previous character for context.
+                let i = cmp::max(v.len(), 1) - 1;
+                let e = u64::from_str_radix(&s[i..], radix).unwrap_err();
                 return Err(e.into());
             }
         }
diff --git a/bigint/src/tests/biguint.rs b/bigint/src/tests/biguint.rs
index 87a223f..01dec42 100644
--- a/bigint/src/tests/biguint.rs
+++ b/bigint/src/tests/biguint.rs
@@ -1041,6 +1041,8 @@ fn test_from_str_radix() {
     assert_eq!(plus_plus_one, None);
     let minus_one = BigUint::from_str_radix("-1", 10).ok();
     assert_eq!(minus_one, None);
+    let zero_plus_two = BigUint::from_str_radix("0+2", 10).ok();
+    assert_eq!(zero_plus_two, None);
 }
 
 #[test]