diff --git a/mesonbuild/backend/ninjabackend.py b/mesonbuild/backend/ninjabackend.py
index d66708c59..a0087955c 100644
--- a/mesonbuild/backend/ninjabackend.py
+++ b/mesonbuild/backend/ninjabackend.py
@@ -637,7 +637,7 @@ int dummy;
         if self.environment.is_llvm_ir(source) or \
            self.environment.is_assembly(source):
             return False
-        suffix = os.path.splitext(source)[1][1:]
+        suffix = os.path.splitext(source)[1][1:].lower()
         for lang in self.langs_cant_unity:
             if lang not in target.compilers:
                 continue
@@ -925,7 +925,7 @@ int dummy;
         all_suffixes = set(compilers.lang_suffixes['cpp']) | set(compilers.lang_suffixes['fortran'])
         selected_sources = []
         for source in compiled_sources:
-            ext = os.path.splitext(source)[1][1:]
+            ext = os.path.splitext(source)[1][1:].lower()
             if ext in all_suffixes:
                 selected_sources.append(source)
         return selected_sources
diff --git a/mesonbuild/scripts/depscan.py b/mesonbuild/scripts/depscan.py
index df7df4889..c85f8e756 100644
--- a/mesonbuild/scripts/depscan.py
+++ b/mesonbuild/scripts/depscan.py
@@ -46,7 +46,7 @@ class DependencyScanner:
         self.sources_with_exports = [] # type: T.List[str]
 
     def scan_file(self, fname: str) -> None:
-        suffix = os.path.splitext(fname)[1][1:]
+        suffix = os.path.splitext(fname)[1][1:].lower()
         if suffix in lang_suffixes['fortran']:
             self.scan_fortran_file(fname)
         elif suffix in lang_suffixes['cpp']:
@@ -131,7 +131,7 @@ class DependencyScanner:
         return objname
 
     def module_name_for(self, src: str) -> str:
-        suffix= os.path.splitext(src)[1][1:]
+        suffix = os.path.splitext(src)[1][1:].lower()
         if suffix in lang_suffixes['fortran']:
             exported = self.exports[src]
             # Module foo:bar goes to a file name foo@bar.smod