diff --git a/pgtype/hstore.go b/pgtype/hstore.go
index 1a83faf55..c58173d35 100644
--- a/pgtype/hstore.go
+++ b/pgtype/hstore.go
@@ -431,8 +431,13 @@ func parseHstore(s string) (Hstore, error) {
 	p := newHSP(s)
 
 	// This is an over-estimate of the number of key/value pairs. Use '>' because I am guessing it
-	// is less likely to occur in keys/values than '=' or ','.
+	// is less likely to occur in keys/values than '=' or ','. Clamp so an unvalidated
+	// separator count cannot pre-size a huge map from garbage input.
+	const maxHstorePairsEstimate = 1024
 	numPairsEstimate := strings.Count(s, ">")
+	if numPairsEstimate > maxHstorePairsEstimate {
+		numPairsEstimate = maxHstorePairsEstimate
+	}
 	// makes one allocation of strings for the entire Hstore, rather than one allocation per value.
 	valueStrings := make([]string, 0, numPairsEstimate)
 	result := make(Hstore, numPairsEstimate)
diff --git a/pgtype/hstore_test.go b/pgtype/hstore_test.go
index e56a6a655..49e3a2173 100644
--- a/pgtype/hstore_test.go
+++ b/pgtype/hstore_test.go
@@ -4,6 +4,7 @@ import (
 	"context"
 	"fmt"
 	"reflect"
+	"strings"
 	"testing"
 	"time"
 
@@ -408,3 +409,29 @@ func BenchmarkHstoreScan(b *testing.B) {
 		})
 	}
 }
+
+func TestHstoreScanGarbageDoesNotPreallocateFromSeparatorCount(t *testing.T) {
+	var h pgtype.Hstore
+	err := h.Scan(strings.Repeat(">", 200000))
+	if err == nil {
+		t.Fatal("expected error scanning invalid hstore text")
+	}
+}
+
+func TestHstoreScanMorePairsThanEstimateClamp(t *testing.T) {
+	const n = 2000
+	var b strings.Builder
+	for i := 0; i < n; i++ {
+		if i > 0 {
+			b.WriteString(", ")
+		}
+		fmt.Fprintf(&b, `"k%d"=>"v%d"`, i, i)
+	}
+	var h pgtype.Hstore
+	if err := h.Scan(b.String()); err != nil {
+		t.Fatal(err)
+	}
+	if len(h) != n {
+		t.Fatalf("len(h) = %d, want %d", len(h), n)
+	}
+}