diff --git a/extern/fftw/CMakeLists.txt b/extern/fftw/CMakeLists.txt
index 369e665d..6b807c48 100644
--- a/extern/fftw/CMakeLists.txt
+++ b/extern/fftw/CMakeLists.txt
@@ -312,7 +312,7 @@ endif ()
 add_library (${fftw3_lib} ${SOURCEFILES})
 target_include_directories (${fftw3_lib} INTERFACE $<INSTALL_INTERFACE:include>)
 if (WITH_OUR_MALLOC)
-  target_compile_options (${fftw3_lib} PRIVATE WITH_OUR_MALLOC)
+  target_compile_definitions (${fftw3_lib} PRIVATE WITH_OUR_MALLOC)
 endif ()
 if (MSVC AND NOT (CMAKE_C_COMPILER_ID STREQUAL "Intel"))
   target_compile_definitions (${fftw3_lib} PRIVATE /bigobj)