diff --git a/port/cpl_multiproc.cpp b/port/cpl_multiproc.cpp
index 65384486c29b..8e620fbab50c 100644
--- a/port/cpl_multiproc.cpp
+++ b/port/cpl_multiproc.cpp
@@ -318,14 +318,14 @@ CPLMutex *CPLCreateOrAcquireMasterMutex(double dfWaitInSeconds = 1000.0)
 
 #ifdef MUTEX_NONE
 
-int CPLCreateOrAcquireMutexEx(CPLMutex **phMutex, double dfWaitInSeconds,
-                              int nOptions)
+bool CPLCreateOrAcquireMutexEx(CPLMutex **phMutex, double dfWaitInSeconds,
+                               int nOptions)
 {
     return false;
 }
 #else
-int CPLCreateOrAcquireMutexEx(CPLMutex **phMutex, double dfWaitInSeconds,
-                              int nOptions)
+bool CPLCreateOrAcquireMutexEx(CPLMutex **phMutex, double dfWaitInSeconds,
+                               int nOptions)
 {
     bool bSuccess = false;