diff --git a/doc/release-notes.md b/doc/release-notes.md
--- a/doc/release-notes.md
+++ b/doc/release-notes.md
@@ -9,4 +9,5 @@
    (previously was `Bitcoin Signed Message:`).
  - A new `getindexinfo` RPC returns the actively running indices of the node,
    including their current sync status and height. It also accepts an `index_name`
-   to specify returning only the status of that index.
\ No newline at end of file
+   to specify returning only the status of that index.
+ - The `getnetworkhashps` RPC no longer accepts a negative number of blocks.
diff --git a/src/rpc/mining.cpp b/src/rpc/mining.cpp
--- a/src/rpc/mining.cpp
+++ b/src/rpc/mining.cpp
@@ -42,10 +42,10 @@
 
 /**
  * Return average network hashes per second based on the last 'lookup' blocks,
- * or from the last difficulty change if 'lookup' is nonpositive. If 'height' is
- * nonnegative, compute the estimate at the time when a given block was found.
+ * If 'height' is nonnegative, compute the estimate at the time when a given
+ * block was found.
  */
-static UniValue GetNetworkHashPS(int lookup, int height) {
+static UniValue GetNetworkHashPS(unsigned int lookup, int height) {
     CBlockIndex *pb = ::ChainActive().Tip();
 
     if (height >= 0 && height < ::ChainActive().Height()) {
@@ -56,22 +56,16 @@
         return 0;
     }
 
-    // If lookup is -1, then use blocks since last difficulty change.
-    if (lookup <= 0) {
-        lookup = pb->nHeight %
-                     Params().GetConsensus().DifficultyAdjustmentInterval() +
-                 1;
-    }
-
+    CHECK_NONFATAL(pb->nHeight > 0);
     // If lookup is larger than chain, then set it to chain length.
-    if (lookup > pb->nHeight) {
+    if (lookup > (unsigned int)pb->nHeight) {
         lookup = pb->nHeight;
     }
 
     CBlockIndex *pb0 = pb;
     int64_t minTime = pb0->GetBlockTime();
     int64_t maxTime = minTime;
-    for (int i = 0; i < lookup; i++) {
+    for (size_t i = 0; i < lookup; i++) {
         pb0 = pb0->pprev;
         int64_t time = pb0->GetBlockTime();
         minTime = std::min(time, minTime);
@@ -96,14 +90,12 @@
         "getnetworkhashps",
         "Returns the estimated network hashes per second based on the last n "
         "blocks.\n"
-        "Pass in [blocks] to override # of blocks, -1 specifies since last "
-        "difficulty change.\n"
+        "Pass in [nblocks] to override # of blocks.\n"
         "Pass in [height] to estimate the network speed at the time when a "
         "certain block was found.\n",
         {
             {"nblocks", RPCArg::Type::NUM, /* default */ "120",
-             "The number of blocks, or -1 for blocks since last difficulty "
-             "change."},
+             "The number of blocks."},
             {"height", RPCArg::Type::NUM, /* default */ "-1",
              "To estimate at the time of the given height."},
         },
@@ -113,10 +105,17 @@
     }
         .Check(request);
 
+    int nblocks =
+        !request.params[0].isNull() ? request.params[0].get_int() : 120;
+    if (nblocks <= 0) {
+        throw JSONRPCError(RPC_INVALID_PARAMETER,
+                           "Number of blocks cannot be less than 1");
+    }
+
     LOCK(cs_main);
-    return GetNetworkHashPS(
-        !request.params[0].isNull() ? request.params[0].get_int() : 120,
-        !request.params[1].isNull() ? request.params[1].get_int() : -1);
+    return GetNetworkHashPS(nblocks, !request.params[1].isNull()
+                                         ? request.params[1].get_int()
+                                         : -1);
 }
 
 static bool GenerateBlock(const Config &config, ChainstateManager &chainman,
diff --git a/test/functional/rpc_blockchain.py b/test/functional/rpc_blockchain.py
--- a/test/functional/rpc_blockchain.py
+++ b/test/functional/rpc_blockchain.py
@@ -337,6 +337,12 @@
         # This should be 2 hashes every 10 minutes or 1/300
         assert abs(hashes_per_second * 300 - 1) < 0.0001
 
+        # Passing nblocks = -1 used to mean "since last difficulty change".
+        # This is now meaningless, as the difficulty adjusts for every block.
+        assert_raises_rpc_error(
+            -8, "Number of blocks cannot be less than 1",
+            self.nodes[0].getnetworkhashps, -1)
+
     def _test_stopatheight(self):
         assert_equal(self.nodes[0].getblockcount(), 200)
         self.nodes[0].generatetoaddress(