diff --git a/src/c_import.zig b/src/c_import.zig
index 98248ae7..b1ec6868 100644
--- a/src/c_import.zig
+++ b/src/c_import.zig
@@ -332,10 +332,20 @@ fn checkForeignRefs(valid: *const std.StringHashMap(void), decls: []const *Node,
     for (decls) |d| {
         switch (d.data) {
             .fn_decl => |fd| {
-                if (fd.body.data != .foreign_expr) continue;
-                const ref = fd.body.data.foreign_expr.library_ref orelse continue;
+                // A library reference rides on the legacy `#foreign` body
+                // (foreign_expr.library_ref) OR the new `extern LIB` keyword
+                // (extern_lib); both must name a declared #library / #import c
+                // unit. The diagnostic names the surface keyword the user wrote
+                // so the two spellings stay self-describing.
+                const kw: []const u8, const ref: []const u8 = switch (fd.body.data) {
+                    .foreign_expr => |fe| .{ "#foreign", fe.library_ref orelse continue },
+                    else => if (fd.extern_export == .extern_)
+                        .{ "extern", fd.extern_lib orelse continue }
+                    else
+                        continue,
+                };
                 if (!valid.contains(ref)) {
-                    diags.addFmt(.err, d.span, "#foreign library '{s}' is not declared; expected a #library constant or a named '#import c' unit", .{ref});
+                    diags.addFmt(.err, d.span, "{s} library '{s}' is not declared; expected a #library constant or a named '#import c' unit", .{ kw, ref });
                 }
             },
             .namespace_decl => |ns| checkForeignRefs(valid, ns.decls, diags),