diff --git a/mesonbuild/modules/external_project.py b/mesonbuild/modules/external_project.py
index 64f7d57d2f30..0b6227435aea 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 = Path(relpath(self.prefix, self.prefix.drive))
+            self.prefix = Path('/' + relpath(self.prefix, self.prefix.drive))
 
         # self.prefix is an absolute path, so we cannot append it to another path.
         self.rel_prefix = Path(relpath(self.prefix, self.prefix.root))