diff --git a/src/compat/assumptions.h b/src/compat/assumptions.h
index 487ab9aa9e873..0298f78b96def 100644
--- a/src/compat/assumptions.h
+++ b/src/compat/assumptions.h
@@ -8,6 +8,7 @@
 #ifndef BITCOIN_COMPAT_ASSUMPTIONS_H
 #define BITCOIN_COMPAT_ASSUMPTIONS_H
 
+#include <cstddef>
 #include <limits>
 
 // Assumption: We assume that the macro NDEBUG is not defined.