diff --git a/docs/ACKNOWLEDGEMENT.md b/docs/ACKNOWLEDGEMENT.md
index 33feb7705..be74d7ca6 100644
--- a/docs/ACKNOWLEDGEMENT.md
+++ b/docs/ACKNOWLEDGEMENT.md
@@ -521,7 +521,7 @@ Parallel Connected-Component Labelling on GPUs" (2018), IEEE Trans. Parallel Dis
 The FFT indexer's GPU peak search divides by a fixed window width with a reciprocal multiplication
 corrected once by the exact remainder, which returns the correctly rounded quotient, following
 P. W. Markstein, "Computation of elementary functions on the IBM RISC System/6000 processor"
-(1990), IBM J. Res. Dev. 34, 111-119 (DOI not verified here).
+(1990), IBM J. Res. Dev. 34, 111-119.
 
 This software uses the Viridis, Magma and Inferno colormaps from Matplotlib under its
 BSD-compatible license. J. D. Hunter, "Matplotlib: A 2D graphics environment" (2007), Comput. Sci.