diff --git a/imgui-sys/build.rs b/imgui-sys/build.rs
index 7c2991a..954ca6d 100644
--- a/imgui-sys/build.rs
+++ b/imgui-sys/build.rs
@@ -1,10 +1,34 @@
-fn main() {
-    cc::Build::new()
-        .cpp(true)
-        .file("third-party/cimgui/cimgui.cpp")
-        .file("third-party/cimgui/imgui/imgui.cpp")
-        .file("third-party/cimgui/imgui/imgui_demo.cpp")
-        .file("third-party/cimgui/imgui/imgui_draw.cpp")
-        .file("third-party/cimgui/imgui/imgui_widgets.cpp")
-        .compile("libcimgui.a");
+use std::fs;
+use std::io;
+
+const CPP_FILES: [&str; 5] = [
+    "third-party/cimgui/cimgui.cpp",
+    "third-party/cimgui/imgui/imgui.cpp",
+    "third-party/cimgui/imgui/imgui_demo.cpp",
+    "third-party/cimgui/imgui/imgui_draw.cpp",
+    "third-party/cimgui/imgui/imgui_widgets.cpp",
+];
+
+fn assert_file_exists(path: &str) -> io::Result<()> {
+    match fs::metadata(path) {
+        Ok(_) => Ok(()),
+        Err(ref e) if e.kind() == io::ErrorKind::NotFound => {
+            panic!(
+                "Can't access {}. Did you forget to fetch git submodules?",
+                path
+            );
+        }
+        Err(e) => Err(e),
+    }
+}
+
+fn main() -> io::Result<()> {
+    let mut build = cc::Build::new();
+    build.cpp(true);
+    for path in &CPP_FILES {
+        assert_file_exists(path)?;
+        build.file(path);
+    }
+    build.compile("libcimgui.a");
+    Ok(())
 }