diff --git a/context.go b/context.go
index a20de91d7aa9cd822467b678a45ea761c757d1ab..c1d5b38aff6780df3f88a693a4ebb82c13e10808 100644
--- a/context.go
+++ b/context.go
@@ -1913,10 +1913,9 @@ func (c *Context) requireNinjaVersion(major, minor, micro int) {
 }
 
 func (c *Context) setBuildDir(value *ninjaString) {
-	if c.buildDir != nil {
-		panic("buildDir set multiple times")
+	if c.buildDir == nil {
+		c.buildDir = value
 	}
-	c.buildDir = value
 }
 
 func (c *Context) makeUniquePackageNames(