Owner's Manual (PDF)
Page 28
... not pair an original memory module with a new memory module. Replacing Memory Module(s) CAUTION: If the memory module is not installed correctly, your computer during a memory upgrade, keep them separate from any new module(s) that you may have, even if you purchased the new module(s) from Dell. The recommended memory configurations are: matched memory modules installed in DIMM connectors...
... not pair an original memory module with a new memory module. Replacing Memory Module(s) CAUTION: If the memory module is not installed correctly, your computer during a memory upgrade, keep them separate from any new module(s) that you may have, even if you purchased the new module(s) from Dell. The recommended memory configurations are: matched memory modules installed in DIMM connectors...