diff --git a/src/Rcpp-adapters/FunctionsAdapter.cpp b/src/Rcpp-adapters/FunctionsAdapter.cpp
index 02d9f0af..5c980ed4 100644
--- a/src/Rcpp-adapters/FunctionsAdapter.cpp
+++ b/src/Rcpp-adapters/FunctionsAdapter.cpp
@@ -360,7 +360,11 @@ namespace exageostat::adapters {
         aConfigurations.SetDenseTileSize(aDenseTileSize);
         aConfigurations.SetLowTileSize(aLowTileSize);
         aConfigurations.SetDimension(validator::Validator::CheckDimensionValue(aDimension));
-        aConfigurations.SetProblemSize(data->GetLocations()->GetSize());
+        if (aConfigurations.GetIsFisher()) {
+            aConfigurations.SetProblemSize(train_data_size);
+        } else {
+            aConfigurations.SetProblemSize(train_data_size + test_data_size);
+        }
         aConfigurations.SetEstimatedTheta(aEstimatedTheta);
 
         // Temporarily release ownership to pass to the function.
diff --git a/tests/cpp-tests/Rcpp-adapters/CMakeLists.txt b/tests/cpp-tests/Rcpp-adapters/CMakeLists.txt
index 9a4a59a5..c80a2cf3 100644
--- a/tests/cpp-tests/Rcpp-adapters/CMakeLists.txt
+++ b/tests/cpp-tests/Rcpp-adapters/CMakeLists.txt
@@ -10,6 +10,7 @@
 set(EXAGEOSTAT_TESTFILES
 
         ${CMAKE_CURRENT_SOURCE_DIR}/TestAllRFunctions.cpp
+        ${CMAKE_CURRENT_SOURCE_DIR}/TestRPredictProblemSize.cpp
         ${EXAGEOSTAT_TESTFILES}
         PARENT_SCOPE
         )
diff --git a/tests/cpp-tests/Rcpp-adapters/TestRPredictProblemSize.cpp b/tests/cpp-tests/Rcpp-adapters/TestRPredictProblemSize.cpp
new file mode 100644
index 00000000..d6d94a21
--- /dev/null
+++ b/tests/cpp-tests/Rcpp-adapters/TestRPredictProblemSize.cpp
@@ -0,0 +1,85 @@
+// Copyright (c) 2017-2024 King Abdullah University of Science and Technology,
+// All rights reserved.
+// ExaGeoStat is a software package, provided by King Abdullah University of Science and Technology (KAUST).
+
+/**
+ * @file TestRPredictProblemSize.cpp
+ * @brief Regression test for the problem-size configured by the R prediction adapter.
+ * @details PredictionSetupHelper (used by predict_data / R_ExaGeoStatPredictData) configured the problem
+ * size from the training locations only:
+ *
+ *     aConfigurations.SetProblemSize(data->GetLocations()->GetSize());   // == number of TRAINING points
+ *
+ * The prediction pipeline, however, works over the combined set of observed (training) and missing (test)
+ * locations, and sizes its internal Z descriptor from the configured problem size. When the number of test
+ * (missing) points is comparable to or larger than the number of training points, the problem size is too
+ * small and the prediction fails / produces incorrect values.
+ *
+ * The existing R-adapter test never triggers this because it uses 16 training points and only 2 test
+ * points (test << train). This test deliberately uses MORE test points than training points and checks the
+ * fundamental kriging interpolation property: predicting at an observed location must return the observed
+ * value (for an exact, nugget-free model). It uses the non-nugget univariate_matern_stationary kernel so it
+ * is independent of any other kernel-specific behaviour.
+ * @version 1.1.0
+**/
+
+#include <vector>
+
+#include <catch2/catch_all.hpp>
+
+#include <Rcpp-adapters/FunctionsAdapter.hpp>
+
+using namespace std;
+
+using namespace exageostat::adapters;
+using namespace exageostat::common;
+
+void TEST_R_PREDICT_PROBLEM_SIZE() {
+
+    const string kernel = "univariate_matern_stationary";
+    const string distance_matrix = "eg";
+    const string dimension = "2D";
+    const int dts = 3;
+    const int lts = 0;
+    const vector<double> estimated_theta = {1.0, 0.1, 0.5};
+
+    // 9 training points with distinct measurements.
+    vector<double> train_x = {0.10, 0.22, 0.31, 0.43, 0.55, 0.62, 0.74, 0.81, 0.93};
+    vector<double> train_y = {0.12, 0.27, 0.39, 0.41, 0.58, 0.63, 0.71, 0.86, 0.95};
+    vector<double> train_z = {-1.0, 2.0, 0.5, -0.5, 1.5, -1.5, 0.8, -0.8, 1.1};
+    vector<vector<double>> train_data = {train_x, train_y, train_z};
+
+    auto hardware = ExaGeoStatHardware("exact", 1, 0);
+
+    SECTION("Prediction with more test points than training points") {
+
+        // 12 test points = the 9 training locations + 3 extra. N_test (12) > N_train (9), so a problem size
+        // computed from the training set alone is too small for the prediction descriptors.
+        vector<double> test_x = train_x;
+        vector<double> test_y = train_y;
+        test_x.push_back(0.18); test_y.push_back(0.20);
+        test_x.push_back(0.50); test_y.push_back(0.50);
+        test_x.push_back(0.88); test_y.push_back(0.90);
+        vector<vector<double>> test_data = {test_x, test_y};
+
+        REQUIRE(test_x.size() > train_x.size());
+
+        auto result = R_ExaGeoStatPredictData(kernel, distance_matrix, estimated_theta, dts, lts, dimension,
+                                              train_data, test_data);
+
+        // One prediction per test point.
+        REQUIRE(result.size() == test_x.size());
+
+        // Interpolation property: the prediction at each training location (the first 9 test points, which
+        // coincide with the training locations) must recover the observed value for an exact, nugget-free
+        // model. With the under-sized problem size the prediction is wrong; with the correct size it holds.
+        for (size_t i = 0; i < train_x.size(); i++) {
+            INFO("training location " << i << " at (" << train_x[i] << ", " << train_y[i] << ")");
+            REQUIRE(result[i] == Catch::Approx(train_z[i]).margin(1e-4));
+        }
+    }
+}
+
+TEST_CASE("R predict_data problem-size regression") {
+    TEST_R_PREDICT_PROBLEM_SIZE();
+}