--- a/src/buildtools/config/linux.pri
+++ b/src/buildtools/config/linux.pri
@@ -48,6 +48,7 @@
 
 cross_compile:!host_build {
     TOOLCHAIN_SYSROOT = $$[QT_SYSROOT]
+    isEmpty(TOOLCHAIN_SYSROOT): TOOLCHAIN_SYSROOT = $$DUMMY_SYSROOT
     !isEmpty(TOOLCHAIN_SYSROOT): gn_args += target_sysroot=\"$${TOOLCHAIN_SYSROOT}\"
 }
 
