diff --git a/image_analysis/spot_finding/ImageSpotFinderCPU.cpp b/image_analysis/spot_finding/ImageSpotFinderCPU.cpp
index 1cb6773f9..4062f5b10 100644
--- a/image_analysis/spot_finding/ImageSpotFinderCPU.cpp
+++ b/image_analysis/spot_finding/ImageSpotFinderCPU.cpp
@@ -45,14 +45,15 @@ bool StrongInWindow(int64_t pxl_val, int64_t sum, int64_t sum2, int64_t valid,
                 // pixel is above SNR threshold
 }
 
-// Adds row `in` to the vertical sums and takes row `out` out of them; nullptr is no row. A bad or
-// saturated pixel adds 0. Written without branches, and as a function of its own so that the loop
-// vectorises whatever it is inlined into; integer sums, so the same totals as pixel by pixel.
+// Adds row `in` to the vertical sums and takes row `out` out of them; a row of INT32_MIN is no row. A
+// bad or saturated pixel adds 0. Written without branches - both rows are always read, so no load is
+// conditional - and as a function of its own, so that the loop vectorises whatever it is inlined into;
+// integer sums, so the same totals as pixel by pixel.
 void SlideVertical(const int32_t *in, const int32_t *out, int32_t width,
                    int64_t *sum, int64_t *sum2, uint16_t *valid) {
     for (int32_t col = 0; col < width; col++) {
-        const int64_t a = in ? in[col] : INT32_MIN;
-        const int64_t r = out ? out[col] : INT32_MIN;
+        const int64_t a = in[col];
+        const int64_t r = out[col];
         const bool a_ok = a != INT32_MAX && a != INT32_MIN;
         const bool r_ok = r != INT32_MAX && r != INT32_MIN;
         const int64_t a_v = a_ok ? a : 0;
@@ -190,9 +191,10 @@ void ImageSpotFinderCPU::DetectPass(const ImagePreprocessorBuffer &image,
         int64_t *sv = sum_vert.data();
         int64_t *sv2 = sum2_vert.data();
         uint16_t *vv = valid_vert.data();
+        const std::vector<int32_t> no_row(width, INT32_MIN);
         auto slide_vert = [&](int line_in, int line_out) {
-            SlideVertical(line_in >= 0 ? img + static_cast<size_t>(line_in) * w : nullptr,
-                          line_out >= 0 ? img + static_cast<size_t>(line_out) * w : nullptr, w, sv, sv2, vv);
+            SlideVertical(line_in >= 0 ? img + static_cast<size_t>(line_in) * w : no_row.data(),
+                          line_out >= 0 ? img + static_cast<size_t>(line_out) * w : no_row.data(), w, sv, sv2, vv);
             if (!prev_strong)
                 return;
             // sign = +1 takes out what line_in added, -1 puts back what line_out took out.