Assuming no essential difference between bilingual and monolingual linguistic capacity, the present study offers a Null Theory of intra-sentential CS in terms of Chomsky’s (2000, 2001 and 2008) phase theory with negative and positive evidence from Urdu/English code-switching. The inability of the existing CS-models in accounting for the recurring switches in the data necessitates the minimalist account of CS offered in the study. It is proposed that both mixed and unmixed sentences are derived in a similar fashion in two distinct derivational chunks called Phases. Since one phase does not remain accessible to the other due to Phase Impenetrability Condition, switches in each phase are determined independently of each other. As locus of parametric variation, v plays crucial role in the interaction between two distinct grammatical systems. As documented in the data, selection of v from one lexicon precludes the selection of T and D from the opposite lexicon due to a mismatch in their feature specifications; however, C, being phase head itself remains out of the control of v, and therefore, may be contributed by either of the lexicons subject to its compatibility with its complement TP. Adopting ‘root’ view of lexical categories, it is argued that lexical categories being unspecified roots may be supplied by either of the lexicons randomly; however, selection of functional categories is subject to their correspondence to the feature specifications of v. The difference in switching behavior of adjunct and complement projections also stems from their different structural relation to v. It is further posited that it is v instead of lexical head or head-parameter which determines the word-order of the constituents. It is concluded that all mixed and unmixed derivations including the switches involving different heads and complements adhere to a general condition of well-formedness which restricts any conflict in feature specifications of the syntactic objects involved in a derivation; hence no CS-specific postulates required to account for CS-data.