diff --git a/FindFsp.cmake b/FindFsp.cmake
index dd4f518..bf0f2bc 100644
--- a/FindFsp.cmake
+++ b/FindFsp.cmake
@@ -4,11 +4,11 @@ if(NOT FSP_IS_EXTERNAL_PROJECT)
   set(FSP_LIBRARY_DIR "" CACHE PATH "The path to the library of an libfsp installation")
 
   set(_search_paths "")
-  if(EXISTS FSP_INCLUDE_DIR)
+  if(FSP_INCLUDE_DIR AND EXISTS ${FSP_INCLUDE_DIR})
     list(APPEND _search_paths ${FSP_INCLUDE_DIR})
   endif()
-  if(EXISTS FSP_PREFIX)
-    list(APPEND _search_paths ${FSP_PREFIX}/include)
+  if(FSP_PREFIX AND EXISTS ${FSP_PREFIX})
+    list(APPEND _search_paths "${FSP_PREFIX}/include")
   endif()
   find_path(
     FSP_INCLUDE_DIRS
@@ -16,11 +16,11 @@ if(NOT FSP_IS_EXTERNAL_PROJECT)
     PATHS ${_search_paths})
 
   set(_search_paths "")
-  if(EXISTS FSP_LIBRARY_DIR)
+  if(FSP_LIBRARY_DIR AND EXISTS ${FSP_LIBRARY_DIR})
     list(APPEND _search_paths ${FSP_LIBRARY_DIR})
   endif()
-  if(EXISTS FSP_PREFIX)
-    list(APPEND _search_paths ${FSP_PREFIX}/lib)
+  if(FSP_PREFIX AND EXISTS ${FSP_PREFIX})
+    list(APPEND _search_paths "${FSP_PREFIX}/lib")
   endif()
   find_library(
     FSP_LIBRARIES
diff --git a/FindIonobase.cmake b/FindIonobase.cmake
index b54c560..0645d47 100644
--- a/FindIonobase.cmake
+++ b/FindIonobase.cmake
@@ -4,11 +4,11 @@ if(NOT IONOBASE_IS_EXTERNAL_PROJECT)
   set(IONOBASE_LIBRARY_DIR "" CACHE PATH "The path to the library of an libionobase installation")
 
   set(_search_paths "")
-  if(EXISTS IONOBASE_INCLUDE_DIR)
+  if(IONOBASE_INCLUDE_DIR AND EXISTS ${IONOBASE_INCLUDE_DIR})
     list(APPEND _search_paths ${IONOBASE_INCLUDE_DIR})
   endif()
-  if(EXISTS IONOBASE_PREFIX)
-    list(APPEND _search_paths ${IONOBASE_PREFIX}/include)
+  if(IONOBASE_PREFIX AND EXISTS ${IONOBASE_PREFIX})
+    list(APPEND _search_paths "${IONOBASE_PREFIX}/include")
   endif()
   find_path(
     IONOBASE_INCLUDE_DIRS
@@ -16,11 +16,11 @@ if(NOT IONOBASE_IS_EXTERNAL_PROJECT)
     PATHS ${_search_paths})
 
   set(_search_paths "")
-  if(EXISTS IONOBASE_LIBRARY_DIR)
+  if(IONOBASE_LIBRARY_DIR AND EXISTS ${IONOBASE_LIBRARY_DIR})
     list(APPEND _search_paths ${IONOBASE_LIBRARY_DIR})
   endif()
-  if(EXISTS IONOBASE_PREFIX)
-    list(APPEND _search_paths ${IONOBASE_PREFIX}/lib)
+  if(IONOBASE_PREFIX AND EXISTS ${IONOBASE_PREFIX})
+    list(APPEND _search_paths "${IONOBASE_PREFIX}/lib")
   endif()
   find_library(
     IONOBASE_LIBRARIES
diff --git a/FindPreprocmath.cmake b/FindPreprocmath.cmake
index 935673b..81c7600 100644
--- a/FindPreprocmath.cmake
+++ b/FindPreprocmath.cmake
@@ -4,11 +4,11 @@ if(NOT PREPROCMATH_IS_EXTERNAL_PROJECT)
   set(PREPROCMATH_LIBRARY_DIR "" CACHE PATH "The path to the library of an libpreprocmath installation")
 
   set(_search_paths "")
-  if(EXISTS PREPROCMATH_INCLUDE_DIR)
+  if(PREPROCMATH_INCLUDE_DIR AND EXISTS ${PREPROCMATH_INCLUDE_DIR})
     list(APPEND _search_paths ${PREPROCMATH_INCLUDE_DIR})
   endif()
-  if(EXISTS PREPROCMATH_PREFIX)
-    list(APPEND _search_paths ${PREPROCMATH_PREFIX}/include)
+  if(PREPROCMATH_PREFIX AND EXISTS ${PREPROCMATH_PREFIX})
+    list(APPEND _search_paths "${PREPROCMATH_PREFIX}/include")
   endif()
   find_path(
     PREPROCMATH_INCLUDE_DIRS
@@ -16,11 +16,11 @@ if(NOT PREPROCMATH_IS_EXTERNAL_PROJECT)
     PATHS ${_search_paths})
 
   set(_search_paths "")
-  if(EXISTS PREPROCMATH_LIBRARY_DIR)
+  if(PREPROCMATH_LIBRARY_DIR AND EXISTS ${PREPROCMATH_LIBRARY_DIR})
     list(APPEND _search_paths ${PREPROCMATH_LIBRARY_DIR})
   endif()
-  if(EXISTS PREPROCMATH_PREFIX)
-    list(APPEND _search_paths ${PREPROCMATH_PREFIX}/lib)
+  if(PREPROCMATH_PREFIX AND EXISTS ${PREPROCMATH_PREFIX})
+    list(APPEND _search_paths "${PREPROCMATH_PREFIX}/lib")
   endif()
   find_library(
     PREPROCMATH_LIBRARIES