Index: lld/trunk/MinGW/Driver.cpp
===================================================================
--- lld/trunk/MinGW/Driver.cpp
+++ lld/trunk/MinGW/Driver.cpp
@@ -180,7 +180,7 @@
   for (auto *A : Args) {
     switch (A->getOption().getUnaliasedOption().getID()) {
     case OPT_INPUT:
-      if (StringRef(A->getValue()).endswith(".def"))
+      if (StringRef(A->getValue()).endswith_lower(".def"))
         Add("-def:" + StringRef(A->getValue()));
       else
         Add(Prefix + StringRef(A->getValue()));
Index: lld/trunk/test/MinGW/driver.test
===================================================================
--- lld/trunk/test/MinGW/driver.test
+++ lld/trunk/test/MinGW/driver.test
@@ -26,8 +26,11 @@
 SHARED:      -out:a.dll
 SHARED-SAME: -dll
 
-RUN: ld.lld -### foo.o -m i386pep -shared foo.def | FileCheck -check-prefix=DEF %s
-DEF: -def:foo.def
+RUN: ld.lld -### foo.o -m i386pep -shared foo.def | FileCheck -check-prefix=DEF1 %s
+DEF1: -def:foo.def
+
+RUN: ld.lld -### foo.o -m i386pep -shared FOO.DEF | FileCheck -check-prefix=DEF2 %s
+DEF2: -def:FOO.DEF
 
 RUN: ld.lld -### foo.o -m i386pep -o bar.exe | FileCheck -check-prefix=OUT %s
 OUT: -out:bar.exe