diff --git a/CHANGELOG.rst b/CHANGELOG.rst
index 12dc6070..781d404a 100644
--- a/CHANGELOG.rst
+++ b/CHANGELOG.rst
@@ -10,6 +10,9 @@ latest
 * Tweak Just commands for running version-specific Python tests.
 * Remove `typing-extensions` as a dependency.
 * Make package FIPS compatible by marking blake2 hashing with `usedforsecurity=False`.
+* Raise `ModuleNotPresent` instead of an unclear error when `find_modules_that_directly_import` or
+  `find_modules_directly_imported_by` is called with a module that is not in the graph.
+  (https://github.com/python-grimp/grimp/issues/113)
 
 3.14 (2025-12-10)
 -----------------
diff --git a/src/grimp/application/graph.py b/src/grimp/application/graph.py
index 21533f18..165eecb3 100644
--- a/src/grimp/application/graph.py
+++ b/src/grimp/application/graph.py
@@ -217,13 +217,26 @@ def direct_import_exists(
         )
 
     def find_modules_directly_imported_by(self, module: str) -> set[str]:
+        """
+        Find all modules that are directly imported by the supplied module.
+
+        If the module is not present in the graph, grimp.exceptions.ModuleNotPresent will be raised.
+        """
+        if not self._rustgraph.contains_module(module):
+            msg = f'"{module}" not present in the graph.'
+            raise ModuleNotPresent(msg)
         return self._rustgraph.find_modules_directly_imported_by(module)
 
     def find_modules_that_directly_import(self, module: str) -> set[str]:
-        if self._rustgraph.contains_module(module):
-            # TODO panics if module isn't in modules.
-            return self._rustgraph.find_modules_that_directly_import(module)
-        return set()
+        """
+        Find all modules that directly import the supplied module.
+
+        If the module is not present in the graph, grimp.exceptions.ModuleNotPresent will be raised.
+        """
+        if not self._rustgraph.contains_module(module):
+            msg = f'"{module}" not present in the graph.'
+            raise ModuleNotPresent(msg)
+        return self._rustgraph.find_modules_that_directly_import(module)
 
     def get_import_details(self, *, importer: str, imported: str) -> list[DetailedImport]:
         """
diff --git a/tests/unit/application/graph/test_direct_imports.py b/tests/unit/application/graph/test_direct_imports.py
index c70112ac..dcf58f4e 100644
--- a/tests/unit/application/graph/test_direct_imports.py
+++ b/tests/unit/application/graph/test_direct_imports.py
@@ -3,7 +3,7 @@
 import pytest
 
 from grimp.application.graph import ImportGraph
-from grimp.exceptions import InvalidImportExpression
+from grimp.exceptions import InvalidImportExpression, ModuleNotPresent
 
 
 def test_find_modules_directly_imported_by():
@@ -34,6 +34,22 @@ def test_find_modules_that_directly_import():
     assert {a, f} == graph.find_modules_that_directly_import("bar")
 
 
+def test_find_modules_directly_imported_by_raises_module_not_present():
+    graph = ImportGraph()
+    graph.add_module("foo")
+
+    with pytest.raises(ModuleNotPresent, match=re.escape('"bar" not present in the graph.')):
+        graph.find_modules_directly_imported_by("bar")
+
+
+def test_find_modules_that_directly_import_raises_module_not_present():
+    graph = ImportGraph()
+    graph.add_module("foo")
+
+    with pytest.raises(ModuleNotPresent, match=re.escape('"bar" not present in the graph.')):
+        graph.find_modules_that_directly_import("bar")
+
+
 @pytest.mark.parametrize(
     "importer, imported, as_packages, expected_result",
     (