Rydberg series, rovibrationally isolated by double resonant excitation, are extrapolated to determine rotational-state detailed ionization thresholds to form DCO+ in the bending-excited levels (030) and (040). Analysis of the rovibrational structure derived from the positions of these thresholds for (030) agrees with a simple parameterization extended from a fit to lower vibrational levels. For (040), however, the vibrational angular momentum components (04(0)0) and (04(2)0) are observed to be inverted in energy, with the (04(0)0) component displaced approximately 20 cm(-1) to a position above that of (04(2)0). This perturbation is interpreted to support a conjecture first made by Hirota and co-workers that the vibrational structure of DCO+ is perturbed by a 4:1 bend-stretch Fermi resonance. Extending the pattern by which vibrational angular momentum components are observed to be split in (020) and (030), we establish the unperturbed position of (04(0)0), from which we estimate the matrix element for bend-stretch coupling. Analysis yields a moderate Fermi matrix element, W((04)(0)0)(10(0)0), of 32.43 cm(-1), which is about two-thirds the magnitude of coupling found in the systems, CO2 and NO2+, where bend-stretch mixing significantly effects the character of higher vibrationally excited states.