diff --git a/src/libcamera/process.cpp b/src/libcamera/process.cpp
index f4e4b004c..edbe8664a 100644
--- a/src/libcamera/process.cpp
+++ b/src/libcamera/process.cpp
@@ -241,7 +241,7 @@ int Process::start(const std::string &path,
 	int ret;
 
 	if (running_)
-		return 0;
+		return -EBUSY;
 
 	int childPid = fork();
 	if (childPid == -1) {