diff --git a/CHANGELOG.md b/CHANGELOG.md
index 0615b18f2a..2f9c4a7684 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -69,6 +69,9 @@ Fixed a CMake bug that caused an MPI linking error for our C++ examples in some
 Fixed a bug in `ARKodeSPRKTable_Create` where the coefficient arrays where not
 allocated.
 
+Fix bug on LLP64 platforms (like Windows 64-bit) where `KLU_INDEXTYPE` could be
+32 bits wide even if `SUNDIALS_INT64_T` is defined.
+
 ## Changes to SUNDIALS in release v7.0.0
 
 ### Major Feature
diff --git a/cmake/tpl/FindKLU.cmake b/cmake/tpl/FindKLU.cmake
index 96b9d7343b..a3d817d037 100644
--- a/cmake/tpl/FindKLU.cmake
+++ b/cmake/tpl/FindKLU.cmake
@@ -46,7 +46,7 @@ if (NOT (KLU_INCLUDE_DIR OR KLU_LIBRARY_DIR OR KLU_LIBRARY))
 endif()
 
 # Set library prefixes for Windows
-if(WIN32)
+if(MSVC OR ("${CMAKE_C_SIMULATE_ID}" STREQUAL "MSVC"))
   set(CMAKE_FIND_LIBRARY_PREFIXES lib ${CMAKE_FIND_LIBRARY_PREFIXES})
   set(CMAKE_FIND_LIBRARY_SUFFIXES d.lib ${CMAKE_FIND_LIBRARY_SUFFIXES})
 elseif(APPLE)
@@ -95,7 +95,7 @@ endif ()
 if (NOT SUITESPARSECONFIG_LIBRARY)
     set(SUITESPARSECONFIG_LIBRARY_NAME suitesparseconfig)
     # NOTE: no prefix for this library on windows
-    if(WIN32 AND NOT MSYS)
+    if(MSVC OR ("${CMAKE_C_SIMULATE_ID}" STREQUAL "MSVC"))
         set(CMAKE_FIND_LIBRARY_PREFIXES "")
     endif()
     find_library( SUITESPARSECONFIG_LIBRARY ${SUITESPARSECONFIG_LIBRARY_NAME} ${KLU_LIBRARY_DIR} NO_DEFAULT_PATH)
diff --git a/doc/shared/RecentChanges.rst b/doc/shared/RecentChanges.rst
index d6d4f2d78a..a86448f38d 100644
--- a/doc/shared/RecentChanges.rst
+++ b/doc/shared/RecentChanges.rst
@@ -69,3 +69,6 @@ Fixed a CMake bug that caused an MPI linking error for our C++ examples in some
 
 Fixed a bug in :c:func:`ARKodeSPRKTable_Create` where the coefficient arrays
 where not allocated.
+
+Fix bug on LLP64 platforms (like Windows 64-bit) where ``KLU_INDEXTYPE`` could be
+32 bits wide even if ``SUNDIALS_INT64_T`` is defined.
diff --git a/src/sunlinsol/klu/sunlinsol_klu.c b/src/sunlinsol/klu/sunlinsol_klu.c
index cbdaa54135..568dc1d549 100644
--- a/src/sunlinsol/klu/sunlinsol_klu.c
+++ b/src/sunlinsol/klu/sunlinsol_klu.c
@@ -16,6 +16,7 @@
  * the SUNLINSOL package.
  * -----------------------------------------------------------------*/
 
+#include <stdint.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <sundials/sundials_math.h>
@@ -49,7 +50,7 @@
  */
 
 #if defined(SUNDIALS_INT64_T)
-#define KLU_INDEXTYPE long int
+#define KLU_INDEXTYPE int64_t
 #else
 #define KLU_INDEXTYPE int
 #endif