diff --git a/changelogs/fragments/448-docker_stack-error.yml b/changelogs/fragments/448-docker_stack-error.yml
new file mode 100644
index 00000000..18d11d47
--- /dev/null
+++ b/changelogs/fragments/448-docker_stack-error.yml
@@ -0,0 +1,2 @@
+bugfixes:
+  - "docker_stack - fix broken string formatting when reporting error in case ``compose`` was containing invalid values (https://github.com/ansible-collections/community.docker/pull/448)."
diff --git a/plugins/modules/docker_stack.py b/plugins/modules/docker_stack.py
index c9fc543c..6558734b 100644
--- a/plugins/modules/docker_stack.py
+++ b/plugins/modules/docker_stack.py
@@ -248,8 +248,7 @@ def main():
             elif isinstance(compose_def, string_types):
                 compose_files.append(compose_def)
             else:
-                module.fail_json(msg="compose element '%s' must be a " +
-                                 "string or a dictionary" % compose_def)
+                module.fail_json(msg="compose element '%s' must be a string or a dictionary" % compose_def)
 
         before_stack_services = docker_stack_inspect(module, name)