diff --git a/go/ssa/interp/ops.go b/go/ssa/interp/ops.go
index 86aaee0e..b1fca002 100644
--- a/go/ssa/interp/ops.go
+++ b/go/ssa/interp/ops.go
@@ -952,11 +952,13 @@ func callBuiltin(caller *frame, callpos token.Pos, fn *ssa.Builtin, args []value
 		// append([]T, ...[]T) []T
 		return append(args[0].([]value), args[1].([]value)...)
 
-	case "copy": // copy([]T, []T) int
-		if _, ok := args[1].(string); ok {
-			panic("copy([]byte, string) not yet implemented")
+	case "copy": // copy([]T, []T) int or copy([]byte, string) int
+		src := args[1]
+		if _, ok := src.(string); ok {
+			params := fn.Type().(*types.Signature).Params()
+			src = conv(params.At(0).Type(), params.At(1).Type(), src)
 		}
-		return copy(args[0].([]value), args[1].([]value))
+		return copy(args[0].([]value), src.([]value))
 
 	case "close": // close(chan T)
 		close(args[0].(chan value))
diff --git a/go/types/builtins.go b/go/types/builtins.go
index aca43225..cc048e3a 100644
--- a/go/types/builtins.go
+++ b/go/types/builtins.go
@@ -302,12 +302,11 @@ func (check *Checker) builtin(x *operand, call *ast.CallExpr, id builtinId) (_ b
 			return
 		}
 
+		if check.Types != nil {
+			check.recordBuiltinType(call.Fun, makeSig(Typ[Int], x.typ, y.typ))
+		}
 		x.mode = value
 		x.typ = Typ[Int]
-		if check.Types != nil {
-			S := NewSlice(dst)
-			check.recordBuiltinType(call.Fun, makeSig(x.typ, S, S))
-		}
 
 	case _Delete:
 		// delete(m, k)
diff --git a/go/types/builtins_test.go b/go/types/builtins_test.go
index ca3bac88..8afcb6ba 100644
--- a/go/types/builtins_test.go
+++ b/go/types/builtins_test.go
@@ -50,7 +50,10 @@ var builtinCalls = []struct {
 	{"complex", `type F64 float64; var re, im F64; _ = complex(re, im)`, `func(p.F64, p.F64) complex128`},
 
 	{"copy", `var src, dst []byte; copy(dst, src)`, `func([]byte, []byte) int`},
-	{"copy", `type T [][]int; var src, dst T; _ = copy(dst, src)`, `func([][]int, [][]int) int`},
+	{"copy", `type T [][]int; var src, dst T; _ = copy(dst, src)`, `func(p.T, p.T) int`},
+	{"copy", `var src string; var dst []byte; copy(dst, src)`, `func([]byte, string) int`},
+	{"copy", `type T string; type U []byte; var src T; var dst U; copy(dst, src)`, `func(p.U, p.T) int`},
+	{"copy", `var dst []byte; copy(dst, "hello")`, `func([]byte, string) int`},
 
 	{"delete", `var m map[string]bool; delete(m, "foo")`, `func(map[string]bool, string)`},
 	{"delete", `type (K string; V int); var m map[K]V; delete(m, "foo")`, `func(map[p.K]p.V, p.K)`},