diff --git a/common/src/format.rs b/common/src/format.rs
index 6f6fc661e..5ebbe107d 100644
--- a/common/src/format.rs
+++ b/common/src/format.rs
@@ -608,7 +608,8 @@ impl FormatSpec {
         .map_err(|msg| msg.to_owned())
     }
 
-    pub fn format_string(&self, s: &BorrowedStr) -> Result<String, &'static str> {
+    pub fn format_string(&self, s: &BorrowedStr) -> Result<String, String> {
+        self.validate_format(FormatType::String)?;
         match self.format_type {
             Some(FormatType::String) | None => self
                 .format_sign_and_align(s, "", FormatAlign::Left)
@@ -617,8 +618,15 @@ impl FormatSpec {
                         value.truncate(precision);
                     }
                     value
-                }),
-            _ => Err("Unknown format code for object of type 'str'"),
+                })
+                .map_err(|msg| msg.to_owned()),
+            _ => {
+                let ch = char::from(self.format_type.as_ref().unwrap());
+                Err(format!(
+                    "Unknown format code '{}' for object of type 'str'",
+                    ch
+                ))
+            }
         }
     }
 
diff --git a/vm/src/builtins/str.rs b/vm/src/builtins/str.rs
index 443f61502..83e2581d5 100644
--- a/vm/src/builtins/str.rs
+++ b/vm/src/builtins/str.rs
@@ -730,9 +730,11 @@ impl PyStr {
 
     #[pymethod(name = "__format__")]
     fn format_str(&self, spec: PyStrRef, vm: &VirtualMachine) -> PyResult<String> {
-        FormatSpec::parse(spec.as_str())
-            .and_then(|format_spec| format_spec.format_string(self.borrow()))
-            .map_err(|err| vm.new_value_error(err.to_string()))
+        let format_spec =
+            FormatSpec::parse(spec.as_str()).map_err(|err| vm.new_value_error(err.to_string()))?;
+        format_spec
+            .format_string(self.borrow())
+            .map_err(|msg| vm.new_value_error(msg))
     }
 
     /// Return a titlecased version of the string where words start with an