diff --git a/glib/src/gstring_builder.rs b/glib/src/gstring_builder.rs
index 40f6a15d0476..f6a44f9bc4d7 100644
--- a/glib/src/gstring_builder.rs
+++ b/glib/src/gstring_builder.rs
@@ -29,7 +29,7 @@ wrapper! {
             let allocated_len = (*src).allocated_len;
             let inner = ffi::GString {
                 str: ffi::g_malloc(allocated_len) as *mut _,
-                len: 0,
+                len: (*src).len,
                 allocated_len,
             };
             // +1 to also copy the NUL-terminator
@@ -364,4 +364,17 @@ mod tests {
         write!(&mut s, "bla bla {} bla", 123).unwrap();
         assert_eq!(&*s, "bla bla 123 bla");
     }
+
+    #[test]
+    fn ptr() {
+        use crate::{
+            ffi,
+            translate::{FromGlibPtrFull, IntoGlibPtr},
+        };
+
+        let s: crate::GStringBuilder = crate::GStringBuilder::new("This is a string.");
+        let s: *const ffi::GString = s.into_glib_ptr();
+        let s = unsafe { crate::GStringBuilder::from_glib_full(s) };
+        assert_eq!(&*s, "This is a string.");
+    }
 }