diff --git a/alibuild_helpers/utilities.py b/alibuild_helpers/utilities.py
index 9d62dc8a..a78c175e 100644
--- a/alibuild_helpers/utilities.py
+++ b/alibuild_helpers/utilities.py
@@ -493,27 +493,28 @@ def getPackageList(packages, specs, configDir, preferSystem, noSystem,
             if re.match(replacement_matcher, key):
               replacement = spec["prefer_system_replacement_specs"][replacement_matcher]
               break
-          dieOnError(replacement is None, "Could not find named replacement spec for "
-                     "%s: %s" % (spec["package"], key))
-          assert(replacement)
-          # We must keep the package name the same, since it is used to
-          # specify dependencies.
-          replacement["package"] = spec["package"]
-          # The version is required for all specs. What we put there will
-          # influence the package's hash, so allow the user to override it.
-          replacement.setdefault("version", requested_version)
-          spec = replacement
-          # Allows generalising the version based on the actual key provided
-          spec["version"] = spec["version"].replace("%(key)s", key)
-          recipe = replacement.get("recipe", "")
-          # If there's an explicitly-specified recipe, we're still building
-          # the package. If not, aliBuild will still "build" it, but it's
-          # basically instantaneous, so report to the user that we're taking
-          # it from the system.
-          if recipe:
-            ownPackages.add(spec["package"])
+          if replacement:
+            # We must keep the package name the same, since it is used to
+            # specify dependencies.
+            replacement["package"] = spec["package"]
+            # The version is required for all specs. What we put there will
+            # influence the package's hash, so allow the user to override it.
+            replacement.setdefault("version", requested_version)
+            spec = replacement
+            # Allows generalising the version based on the actual key provided
+            spec["version"] = spec["version"].replace("%(key)s", key)
+            recipe = replacement.get("recipe", "")
+            # If there's an explicitly-specified recipe, we're still building
+            # the package. If not, aliBuild will still "build" it, but it's
+            # basically instantaneous, so report to the user that we're taking
+            # it from the system.
+            if recipe:
+              ownPackages.add(spec["package"])
+            else:
+              systemPackages.add(spec["package"])
           else:
-            systemPackages.add(spec["package"])
+            warning(f"Could not find named replacement spec for {spec['package']}: {key}, "
+                    "falling back to building the package ourselves.")
 
     dieOnError(("system_requirement" in spec) and recipe.strip("\n\t "),
                "System requirements %s cannot have a recipe" % spec["package"])
diff --git a/tests/test_packagelist.py b/tests/test_packagelist.py
index 7ca6aa47..4f9cd93f 100644
--- a/tests/test_packagelist.py
+++ b/tests/test_packagelist.py
@@ -154,17 +154,20 @@ def test_replacement_recipe_given(self):
         self.assertNotIn("with-replacement-recipe", systemPkgs)
         self.assertIn("with-replacement-recipe", ownPkgs)
 
-    @mock.patch("alibuild_helpers.utilities.dieOnError")
-    def test_missing_replacement_spec(self, mock_dieOnError):
-        """Check an error is thrown when the replacement spec is not found."""
-        assert_msg = "Could not find named replacement spec for missing-spec: missing_tag"
-        # Change the behaviour from sys.exit to a regular exception. Without it
-        # we don't stop execution properly and other asserts might trigger
-        mock_dieOnError.side_effect = lambda cond, _: (_ for _ in ()).throw(Exception("dieOnError called")) if cond else None
-        with self.assertRaises(Exception, msg=assert_msg) as context:
-            specs, systemPkgs, ownPkgs, failedReqs, validDefaults = \
-                getPackageListWithDefaults(["missing-spec"])
-        self.assertEqual(str(context.exception), "dieOnError called", msg=assert_msg)
+    @mock.patch("alibuild_helpers.utilities.warning")
+    def test_missing_replacement_spec(self, mock_warning):
+        """Check a warning is displayed when the replacement spec is not found."""
+        warning_msg = "falling back to building the package ourselves"
+        warning_exists = False
+        def side_effect(msg, *args, **kwargs):
+            nonlocal warning_exists
+            if warning_msg in str(msg):
+              warning_exists = True
+        mock_warning.side_effect = side_effect
+        specs, systemPkgs, ownPkgs, failedReqs, validDefaults = \
+            getPackageListWithDefaults(["missing-spec"])
+        self.assertTrue(warning_exists)
+
 
 
 @mock.patch("alibuild_helpers.utilities.getRecipeReader", new=MockReader)