diff --git a/src/clib/CMakeLists.txt b/src/clib/CMakeLists.txt
index 851877a83bb..d81ec2edbc4 100644
--- a/src/clib/CMakeLists.txt
+++ b/src/clib/CMakeLists.txt
@@ -165,6 +165,8 @@ foreach(x IN LISTS cppdefs)
   string(APPEND CPPFLAGS " -D${x}")
 endforeach()
 foreach(x IN LISTS includes)
-  string(APPEND CPPFLAGS " -I${x}")
+  if (x)
+    string(APPEND CPPFLAGS " -I${x}")
+  endif()
 endforeach()
 set(CPPFLAGS ${CPPFLAGS} PARENT_SCOPE)
diff --git a/src/flib/CMakeLists.txt b/src/flib/CMakeLists.txt
index 822b2e5afd3..1e357c81e5d 100644
--- a/src/flib/CMakeLists.txt
+++ b/src/flib/CMakeLists.txt
@@ -281,6 +281,8 @@ foreach(x IN LISTS fppdefs)
   string(APPEND FPPFLAGS " -D${x}")
 endforeach()
 foreach(x IN LISTS fincludes)
-  string(APPEND FPPFLAGS " -I${x}")
+  if (x)
+    string(APPEND FPPFLAGS " -I${x}")
+  endif()
 endforeach()
 set(FPPFLAGS ${FPPFLAGS} PARENT_SCOPE)