diff --git a/src/core/include/mp-units/framework/expression_template.h b/src/core/include/mp-units/framework/expression_template.h
index f84bce29..acdf953e 100644
--- a/src/core/include/mp-units/framework/expression_template.h
+++ b/src/core/include/mp-units/framework/expression_template.h
@@ -29,6 +29,7 @@
 
 #ifndef MP_UNITS_IN_MODULE_INTERFACE
 #include <cstdint>
+#include <functional>
 #endif
 
 namespace mp_units {