In spatial audio codings, Interchannel Correlation (ICC) synthesis is implemented in two different ways. One uses both ICC and phase parameters, and the other uses only ICC. In the latter, ICC is estimated as a real part of the normalized cross-correlation coefficient between two channels and thus can result in a negative value. Conventional methods assume that ambient components mixed to two output channels are in anti-phase, while the primary signals are assumed to be in phase. When a negative-valued ICC is encountered, this assumption can cause excessive ambient mixing. To solve this problem, we propose a new ICC synthesis method based on an assumption that the primary signals are in anti-phase when negative ICCs are indicated. We first investigate problematic cases of negative ICC synthesis in the conventional methods. Later, we propose a new upmix matrix that satisfies the assumption for the primary components in a negative ICC environment. The effectiveness of the proposed method was verified by computer simulations and subjective listening tests.