diff --git a/src/math/FGNelderMead.h b/src/math/FGNelderMead.h
index e21053c108019607c9b6f63cb7294a07762d4acb..26346519d964603189f4e24c168bbad09e4c5d8f 100644
--- a/src/math/FGNelderMead.h
+++ b/src/math/FGNelderMead.h
@@ -21,6 +21,7 @@
 
 #include <vector>
 #include <limits>
+#include <cstddef>
 
 namespace JSBSim
 {