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Let M and N be differential graded (DG) modules over a positively graded commutative DG algebra A. We show that the Ext-groups defined in terms of semi-projective resolutions are not in general isomorphic to the Yoneda Ext-groups given in terms of equivalence classes of extensions. On the other hand, we show that these groups are isomorphic when the first DG module is semi-projective.