nodejs/node · error · TypeError

Attempt to merge dict value of unsupported type {v.class

Error message

Attempt to merge dict value of unsupported type {v.__class__.__name__} for key {k}

What it means

The final else in MergeDicts: a dict value 'v' is not one of the supported types (str, int, dict, or list). For example None, float, tuple, set, or a custom object as a dict value raises TypeError naming the type and key.

Source

Thrown at tools/gyp/pylib/gyp/input.py:2378

                        + to[list_base].__class__.__name__
                        + " for key "
                        + list_base
                        + "("
                        + k
                        + ")"
                    )
            else:
                to[list_base] = []

            # Call MergeLists, which will make copies of objects that require it.
            # MergeLists can recurse back into MergeDicts, although this will be
            # to make copies of dicts (with paths fixed), there will be no
            # subsequent dict "merging" once entering a list because lists are
            # always replaced, appended to, or prepended to.
            is_paths = IsPathSection(list_base)
            MergeLists(to[list_base], v, to_file, fro_file, is_paths, append)
        else:
            raise TypeError(
                "Attempt to merge dict value of unsupported type "
                + v.__class__.__name__
                + " for key "
                + k
            )


def MergeConfigWithInheritance(
    new_configuration_dict, build_file, target_dict, configuration, visited
):
    # Skip if previously visited.
    if configuration in visited:
        return

    # Look at this configuration.
    configuration_dict = target_dict["configurations"][configuration]

    # Merge in parents.

View on GitHub (pinned to 1b2de5e052)

Solutions

Make every dict value a str, int, dict, or list.
Replace None expansions with concrete strings or remove the key.
Cast floats/bools to str or int as appropriate.
Inspect generator-emitted dicts for non-standard value types.

Example fix

// before: { 'defines': None }
// after: { 'defines': [] }

Defensive patterns

Strategy: type-guard

Validate before calling

def clean_dict(d):
    for k, v in d.items():
        if type(v) not in (str, int) and not isinstance(v, (dict, list)):
            raise TypeError('unsupported dict value type for %s: %r' % (k, type(v)))

Type guard

def is_mergeable_value(v): return type(v) in (str,int) or isinstance(v, (dict,list)) and type(v) is not bool

Try / catch

try:
    MergeDicts(to, fro, to_file, fro_file)
except TypeError as e:
    if 'unsupported type' in str(e): print('Reduce dict values to str/int/dict/list'); raise

Prevention

Restrict dict values to str/int/dict/list; reject None/bool/float/tuple/set.
Avoid variable expansions that yield None.
Validate generator output types.

When it happens

Trigger: During a dict merge, a value associated with some key k is a type MergeDicts cannot handle — str/int/dict/list are the only accepted value types.

Common situations: A variable expanding to None used as a dict value; floats in settings; tuples/sets accidentally introduced by generators; boolean values (type bool) which are NOT matched by `type(v) in (str,int)` and thus fall through.

Related errors

AI-assisted analysis of nodejs/node@1b2de5e052 (2026-08-13). Data as JSON: /api/errors/839da5f00199db9d. Report an issue: GitHub.