diff --git a/deps/p7zip.mk b/deps/p7zip.mk
index 97c4977c1feec..8ad38b2e8d522 100644
--- a/deps/p7zip.mk
+++ b/deps/p7zip.mk
@@ -23,7 +23,11 @@ $(BUILDDIR)/p7zip-$(P7ZIP_VER)/p7zip-15-Enhanced-encryption-strength.patch-appli
 	cd $(dir $@) && patch -p4 -f < $(SRCDIR)/patches/p7zip-15-Enhanced-encryption-strength.patch
 	echo 1 > $@
 
-$(BUILDDIR)/p7zip-$(P7ZIP_VER)/build-configured: $(BUILDDIR)/p7zip-$(P7ZIP_VER)/p7zip-15-Enhanced-encryption-strength.patch-applied
+$(BUILDDIR)/p7zip-$(P7ZIP_VER)/p7zip-Windows_ErrorMsg.patch-applied: $(BUILDDIR)/p7zip-$(P7ZIP_VER)/p7zip-15-Enhanced-encryption-strength.patch-applied
+	cd $(dir $@) && patch -p0 -f < $(SRCDIR)/patches/p7zip-Windows_ErrorMsg.patch
+	echo 1 > $@
+
+$(BUILDDIR)/p7zip-$(P7ZIP_VER)/build-configured: $(BUILDDIR)/p7zip-$(P7ZIP_VER)/p7zip-Windows_ErrorMsg.patch-applied
 $(BUILDDIR)/p7zip-$(P7ZIP_VER)/build-compiled: $(BUILDDIR)/p7zip-$(P7ZIP_VER)/build-configured
 	$(MAKE) -C $(dir $<) $(MAKE_COMMON) CC="$(CC)" CXX="$(CXX)" 7za
 	echo 1 > $@
diff --git a/deps/patches/p7zip-Windows_ErrorMsg.patch b/deps/patches/p7zip-Windows_ErrorMsg.patch
new file mode 100644
index 0000000000000..71de3e9f59c86
--- /dev/null
+++ b/deps/patches/p7zip-Windows_ErrorMsg.patch
@@ -0,0 +1,33 @@
+This fixes the build with Clang 6.0:
+
+ ../../../../CPP/Windows/ErrorMsg.cpp:24:10: error: case value evaluates to -2147024809, which cannot be narrowed to type 'DWORD' (aka  'unsigned int') [-Wc++11-narrowing]
+     case E_INVALIDARG          : txt = "E_INVALIDARG"; break ;
+          ^
+ ../../../../CPP/Common/MyWindows.h:89:22: note: expanded from macro 'E_INVALIDARG'
+ #define E_INVALIDARG ((HRESULT)0x80070057L)
+                      ^
+
+The HRESULT cast in the macro causes the value to be read as signed int.
+--- CPP/Windows/ErrorMsg.cpp.orig	2015-01-18 18:20:28 UTC
++++ CPP/Windows/ErrorMsg.cpp
+@@ -15,13 +15,13 @@ UString MyFormatMessage(DWORD errorCode)
+ 
+   switch(errorCode) {
+     case ERROR_NO_MORE_FILES   : txt = "No more files"; break ;
+-    case E_NOTIMPL             : txt = "E_NOTIMPL"; break ;
+-    case E_NOINTERFACE         : txt = "E_NOINTERFACE"; break ;
+-    case E_ABORT               : txt = "E_ABORT"; break ;
+-    case E_FAIL                : txt = "E_FAIL"; break ;
+-    case STG_E_INVALIDFUNCTION : txt = "STG_E_INVALIDFUNCTION"; break ;
+-    case E_OUTOFMEMORY         : txt = "E_OUTOFMEMORY"; break ;
+-    case E_INVALIDARG          : txt = "E_INVALIDARG"; break ;
++    case (DWORD)(E_NOTIMPL)             : txt = "E_NOTIMPL"; break ;
++    case (DWORD)(E_NOINTERFACE)         : txt = "E_NOINTERFACE"; break ;
++    case (DWORD)(E_ABORT)               : txt = "E_ABORT"; break ;
++    case (DWORD)(E_FAIL)                : txt = "E_FAIL"; break ;
++    case (DWORD)(STG_E_INVALIDFUNCTION) : txt = "STG_E_INVALIDFUNCTION"; break ;
++    case (DWORD)(E_OUTOFMEMORY)         : txt = "E_OUTOFMEMORY"; break ;
++    case (DWORD)(E_INVALIDARG)          : txt = "E_INVALIDARG"; break ;
+     case ERROR_DIRECTORY          : txt = "Error Directory"; break ;
+     default:
+       txt = strerror(errorCode);