diff --git a/mesonbuild/modules/external_project.py b/mesonbuild/modules/external_project.py
index 9e283e026b36..96237370aca3 100644
--- a/mesonbuild/modules/external_project.py
+++ b/mesonbuild/modules/external_project.py
@@ -93,7 +93,7 @@ def __init__(self,
         # will install files into "c:/bar/c:/foo" which is an invalid path.
         # Work around that issue by removing the drive from prefix.
         if self.prefix.drive:
-            self.prefix = self.prefix.relative_to(self.prefix.drive)
+            self.prefix = Path('/' + str(self.prefix.relative_to(self.prefix.drive + "/")))
 
         # self.prefix is an absolute path, so we cannot append it to another path.
         self.rel_prefix = self.prefix.relative_to(self.prefix.root)