diff --git a/tests/corpus/Cargo.toml b/tests/corpus/Cargo.toml
index a1bdcd5..d558e13 100644
--- a/tests/corpus/Cargo.toml
+++ b/tests/corpus/Cargo.toml
@@ -7,6 +7,10 @@ publish = false
 name = "issue-74"
 path = "issue-74.rs"
 
+[[bin]]
+name = "issue-20"
+path = "issue-20.rs"
+
 [[bin]]
 name = "issue-70"
 path = "issue-70.rs"
diff --git a/tests/corpus/issue-20.rs b/tests/corpus/issue-20.rs
new file mode 100644
index 0000000..7d40a0c
--- /dev/null
+++ b/tests/corpus/issue-20.rs
@@ -0,0 +1,10 @@
+extern "C" {
+    fn malloc(n: usize) -> *mut std::ffi::c_void;
+    fn free(ptr: *mut std::ffi::c_void);
+}
+
+fn main() {
+    let ptr = unsafe { malloc(8) };
+    unsafe { free(ptr) };
+    unsafe { free(ptr) };
+}
diff --git a/tests/regression.rs b/tests/regression.rs
index 2a4d1ab..e213417 100644
--- a/tests/regression.rs
+++ b/tests/regression.rs
@@ -27,6 +27,24 @@ fn duplicate_stack_fields() {
         ));
 }
 
+/// Issue: [#20]
+///
+/// [#20]: https://github.com/jfrimmel/cargo-valgrind/issues/20
+#[test]
+fn invalid_free() {
+    cargo_valgrind()
+        .arg("run")
+        .args(TARGET_CRATE)
+        .arg("--bin=issue-20")
+        .assert()
+        .failure()
+        .stderr(predicates::str::contains("Error Invalid free"))
+        .stderr(predicates::str::contains(
+            "is 0 bytes inside a block of size 8 free'd",
+        ))
+        .stderr(predicates::str::contains("Info Block was alloc'd at"));
+}
+
 /// Issue: [#70]
 ///
 /// [#70]: https://github.com/jfrimmel/cargo-valgrind/issues/70