diff --git a/src/ape/managers/networks.py b/src/ape/managers/networks.py
index 320af9739d..68d33287e1 100644
--- a/src/ape/managers/networks.py
+++ b/src/ape/managers/networks.py
@@ -197,12 +197,24 @@ def ecosystems(self) -> dict[str, EcosystemAPI]:
         for custom_network in custom_networks:
             ecosystem_name = custom_network["ecosystem"]
             if ecosystem_name in plugin_ecosystems:
-                # Already included in previous network.
+                # Already included in a prior network.
                 continue
 
             base_ecosystem_name = (
                 custom_network.get("base_ecosystem_plugin") or self.default_ecosystem_name
             )
+
+            if base_ecosystem_name not in plugin_ecosystems:
+                name = custom_network.get("name", "?")
+                if eco := custom_network.get("ecosystem"):
+                    name = f"{eco}:{name}"
+
+                msg = (
+                    f"Custom network '{name}' specified unknown base-ecosystem class "
+                    f"'{base_ecosystem_name}'. Are you missing plugin 'ape-{base_ecosystem_name}'?"
+                )
+                raise NetworkError(msg)
+
             existing_cls = plugin_ecosystems[base_ecosystem_name]
             ecosystem_cls = existing_cls.model_copy(
                 update={"name": ecosystem_name}, cache_clear=("_networks_from_plugins",)
diff --git a/tests/functional/test_network_manager.py b/tests/functional/test_network_manager.py
index 38e64e4c6a..daf5215e7b 100644
--- a/tests/functional/test_network_manager.py
+++ b/tests/functional/test_network_manager.py
@@ -290,6 +290,27 @@ def test_ecosystems_include_custom(networks, custom_networks_config_dict, projec
     assert "custom-ecosystem" in actual
 
 
+def test_ecosystems_when_custom_has_bad_base_ecosystem(
+    networks, custom_networks_config_dict, project
+):
+    data = copy.deepcopy(custom_networks_config_dict)
+    eco_name = "custom-ecosystem-bad"
+    eco_plugin_name = "plugin-ecosystem-not-installed"
+    # Only hits error if ecosystem is custom because it is unknown.
+    data["networks"]["custom"][0]["ecosystem"] = eco_name
+    # The plugin base is for the class-implementation. If it is not a real plugin,
+    # it should fail nicely.
+    data["networks"]["custom"][0]["base_ecosystem_plugin"] = eco_plugin_name
+    expected = (
+        rf"Custom network '{eco_name}:apenet' specified unknown "
+        rf"base-ecosystem class '{eco_plugin_name}'\. "
+        rf"Are you missing plugin 'ape-{eco_plugin_name}'\?"
+    )
+    with project.temp_config(**data):
+        with pytest.raises(NetworkError, match=expected):
+            _ = networks.ecosystems
+
+
 def test_fork_network_not_forkable(networks, eth_tester_provider):
     """
     Show correct failure when trying to fork the local network.