diff --git a/call.go b/call.go
index b2590e97b..f73b7d552 100644
--- a/call.go
+++ b/call.go
@@ -29,7 +29,7 @@ import (
 func (cc *ClientConn) Invoke(ctx context.Context, method string, args, reply interface{}, opts ...CallOption) error {
 	// allow interceptor to see all applicable call options, which means those
 	// configured as defaults from dial option as well as per-call options
-	opts = append(cc.dopts.callOptions, opts...)
+	opts = combine(cc.dopts.callOptions, opts)
 
 	if cc.dopts.unaryInt != nil {
 		return cc.dopts.unaryInt(ctx, method, args, reply, cc, invoke, opts...)
@@ -37,6 +37,21 @@ func (cc *ClientConn) Invoke(ctx context.Context, method string, args, reply int
 	return invoke(ctx, method, args, reply, cc, opts...)
 }
 
+func combine(o1 []CallOption, o2 []CallOption) []CallOption {
+	// we don't use append because o1 could have extra capacity whose
+	// elements would be overwritten, which could cause inadvertent
+	// sharing (and race connditions) between concurrent calls
+	if len(o1) == 0 {
+		return o2
+	} else if len(o2) == 0 {
+		return o1
+	}
+	ret := make([]CallOption, len(o1)+len(o2))
+	copy(ret, o1)
+	copy(ret[len(o1):], o2)
+	return ret
+}
+
 // Invoke sends the RPC request on the wire and returns after response is
 // received.  This is typically called by generated code.
 //
diff --git a/stream.go b/stream.go
index ff376cb01..a79f385a7 100644
--- a/stream.go
+++ b/stream.go
@@ -104,7 +104,7 @@ type ClientStream interface {
 func (cc *ClientConn) NewStream(ctx context.Context, desc *StreamDesc, method string, opts ...CallOption) (ClientStream, error) {
 	// allow interceptor to see all applicable call options, which means those
 	// configured as defaults from dial option as well as per-call options
-	opts = append(cc.dopts.callOptions, opts...)
+	opts = combine(cc.dopts.callOptions, opts)
 
 	if cc.dopts.streamInt != nil {
 		return cc.dopts.streamInt(ctx, desc, cc, method, newClientStream, opts...)