diff --git a/FindMPFR.cmake b/FindMPFR.cmake
index 55ec78e..cf4b701 100644
--- a/FindMPFR.cmake
+++ b/FindMPFR.cmake
@@ -10,14 +10,13 @@
 
 set(MPFR_PREFIX "" CACHE PATH "The path to the prefix of an MPFR installation")
 
-find_path(MPFR_INCLUDE_DIR mpfr.h
+find_path(MPFR_INCLUDE_DIRS mpfr.h
   PATHS ${MPFR_PREFIX}/include)
 
 find_library(MPFR_LIBRARIES NAMES mpfr
   PATHS ${MPFR_PREFIX}/lib)
 
-if(MPFR_INCLUDE_DIR AND MPFR_LIBRARIES)
-  get_filename_component(MPFR_LIBRARY_DIR ${MPFR_LIBRARIES} PATH)
+if(MPFR_INCLUDE_DIRS AND MPFR_LIBRARIES)
   set(MPFR_FOUND TRUE)
 endif()
 
diff --git a/FindPreproc.cmake b/FindPreproc.cmake
index 39410c9..0f26bc0 100644
--- a/FindPreproc.cmake
+++ b/FindPreproc.cmake
@@ -4,11 +4,11 @@ if(NOT PREPROC_IS_EXTERNAL_PROJECT)
   set(PREPROC_LIBRARY_DIR "" CACHE PATH "The path to the library of an libpreproc installation")
 
   set(_search_paths "")
-  if(EXISTS PREPROC_INCLUDE_DIR)
+  if(PREPROC_INCLUDE_DIR AND EXISTS ${PREPROC_INCLUDE_DIR})
     list(APPEND _search_paths ${PREPROC_INCLUDE_DIR})
   endif()
-  if(EXISTS PREPROC_PREFIX)
-    list(APPEND _search_paths ${PREPROC_PREFIX}/include)
+  if(PREPROC_PREFIX AND EXISTS ${PREPROC_PREFIX})
+    list(APPEND _search_paths "${PREPROC_PREFIX}/include")
   endif()
   find_path(
     PREPROC_INCLUDE_DIRS
@@ -16,11 +16,11 @@ if(NOT PREPROC_IS_EXTERNAL_PROJECT)
     PATHS ${_search_paths})
 
   set(_search_paths "")
-  if(EXISTS PREPROC_LIBRARY_DIR)
+  if(PREPROC_LIBRARY_DIR AND EXISTS ${PREPROC_LIBRARY_DIR})
     list(APPEND _search_paths ${PREPROC_LIBRARY_DIR})
   endif()
-  if(EXISTS PREPROC_PREFIX)
-    list(APPEND _search_paths ${PREPROC_PREFIX}/lib)
+  if(PREPROC_PREFIX AND EXISTS ${PREPROC_PREFIX})
+    list(APPEND _search_paths "${PREPROC_PREFIX}/lib")
   endif()
   find_library(
     PREPROC_LIBRARIES