diff --git a/src/pkg/syscall/dll_windows.go b/src/pkg/syscall/dll_windows.go
index 09111ab75c..d29e9921cf 100644
--- a/src/pkg/syscall/dll_windows.go
+++ b/src/pkg/syscall/dll_windows.go
@@ -114,8 +114,14 @@ func (p *Proc) Addr() uintptr {
 	return p.addr
 }
 
-// Call executes procedure p with arguments a.
-func (p *Proc) Call(a ...uintptr) (r1, r2 uintptr, err error) {
+// Call executes procedure p with arguments a. It will panic, if more then 15 arguments
+// are supplied.
+//
+// The returned error is always non-nil, constructed from the result of GetLastError.
+// Callers must inspect the primary return value to decide whether an error occurred
+// (according to the semantics of the specific function being called) before consulting
+// the error. The error will be guaranteed to contain syscall.Errno.
+func (p *Proc) Call(a ...uintptr) (r1, r2 uintptr, lastErr error) {
 	switch len(a) {
 	case 0:
 		return Syscall(p.Addr(), uintptr(len(a)), 0, 0, 0)
@@ -260,8 +266,14 @@ func (p *LazyProc) Addr() uintptr {
 	return p.proc.Addr()
 }
 
-// Call executes procedure p with arguments a.
-func (p *LazyProc) Call(a ...uintptr) (r1, r2 uintptr, err error) {
+// Call executes procedure p with arguments a. It will panic, if more then 15 arguments
+// are supplied.
+//
+// The returned error is always non-nil, constructed from the result of GetLastError.
+// Callers must inspect the primary return value to decide whether an error occurred
+// (according to the semantics of the specific function being called) before consulting
+// the error. The error will be guaranteed to contain syscall.Errno.
+func (p *LazyProc) Call(a ...uintptr) (r1, r2 uintptr, lastErr error) {
 	p.mustFind()
 	return p.proc.Call(a...)
 }