diff --git a/double_interval.h b/double_interval.h
index 6eac4d76..fed7e0ea 100644
--- a/double_interval.h
+++ b/double_interval.h
@@ -32,6 +32,13 @@ typedef struct
 }
 di_t;
 
+#define DI_CHECK(__x) \
+    if (!(__x.a <= __x.b)) \
+    { \
+        flint_printf("di_t endpoints %g, %g not ordered\n", __x.a, __x.b); \
+        flint_abort(); \
+    } \
+
 DOUBLE_INTERVAL_INLINE
 di_t di_interval(double a, double b)
 {
@@ -53,12 +60,22 @@ double _di_below(double x)
 {
     double t;
 
-    t = x;
-    if (t < 0.0)
-        t = -t;
+    if (x <= 1e300)
+    {
+        t = x;
+        if (t < 0.0)
+            t = -t;
 
-    t += 1e-300;
-    return x - t * 4.440892098500626e-16;
+        t += 1e-300;
+        return x - t * 4.440892098500626e-16;
+    }
+    else
+    {
+        if (x != x)
+            return -D_INF;
+
+        return 1e300;
+    }
 }
 
 DOUBLE_INTERVAL_INLINE
@@ -66,12 +83,22 @@ double _di_above(double x)
 {
     double t;
 
-    t = x;
-    if (t < 0.0)
-        t = -t;
+    if (x >= -1e300)
+    {
+        t = x;
+        if (t < 0.0)
+            t = -t;
 
-    t += 1e-300;
-    return x + t * 4.440892098500626e-16;
+        t += 1e-300;
+        return x + t * 4.440892098500626e-16;
+    }
+    else
+    {
+        if (x != x)
+            return D_INF;
+
+        return -1e300;
+    }
 }
 
 DOUBLE_INTERVAL_INLINE
diff --git a/double_interval/fast_mul.c b/double_interval/fast_mul.c
index 6da661f8..9abf63fe 100644
--- a/double_interval/fast_mul.c
+++ b/double_interval/fast_mul.c
@@ -15,22 +15,22 @@ di_t di_fast_mul(di_t x, di_t y)
 {
     di_t res;
 
-    if (x.a >= 0 && y.a >= 0)
+    if (x.a > 0 && y.a > 0)
     {
         res.a = x.a * y.a;
         res.b = x.b * y.b;
     }
-    else if (x.a >= 0 && y.b <= 0)
+    else if (x.a > 0 && y.b < 0)
     {
         res.a = x.b * y.a;
         res.b = x.a * y.b;
     }
-    else if (x.b <= 0 && y.a >= 0)
+    else if (x.b < 0 && y.a > 0)
     {
         res.a = x.a * y.b;
         res.b = x.b * y.a;
     }
-    else if (x.b <= 0 && y.b <= 0)
+    else if (x.b < 0 && y.b < 0)
     {
         res.a = x.b * y.b;
         res.b = x.a * y.a;
diff --git a/double_interval/test/t-fast_add.c b/double_interval/test/t-fast_add.c
index 51c318eb..d820eb8d 100644
--- a/double_interval/test/t-fast_add.c
+++ b/double_interval/test/t-fast_add.c
@@ -37,6 +37,8 @@ int main()
 
         z = di_fast_add(x, y);
 
+        DI_CHECK(z)
+
         arf_set_d(a, x.a);
         arf_set_d(t, y.a);
         arf_add(a, a, t, ARF_PREC_EXACT, ARF_RND_DOWN);
diff --git a/double_interval/test/t-fast_div.c b/double_interval/test/t-fast_div.c
index 85d07bc7..6b85b9c6 100644
--- a/double_interval/test/t-fast_div.c
+++ b/double_interval/test/t-fast_div.c
@@ -58,6 +58,8 @@ int main()
         y = di_randtest(state);
         z = di_fast_div(x, y);
 
+        DI_CHECK(z)
+
         arf_set_d(a, x.a);
         arf_set_d(b, x.b);
         arf_set_d(c, y.a);
diff --git a/double_interval/test/t-fast_mul.c b/double_interval/test/t-fast_mul.c
index d12d86b1..92596839 100644
--- a/double_interval/test/t-fast_mul.c
+++ b/double_interval/test/t-fast_mul.c
@@ -54,10 +54,23 @@ int main()
         arf_init(za);
         arf_init(zb);
 
-        x = di_randtest(state);
-        y = di_randtest(state);
+        if (iter == 0)
+        {
+            x.a = 3.5835915908219665e+103;
+            x.b = 4.0874812242073031e+295;
+            y.a = -8.3711609938763647e+298;
+            y.b = -3.414037107833399e+243;
+        }
+        else
+        {
+            x = di_randtest(state);
+            y = di_randtest(state);
+        }
+
         z = di_fast_mul(x, y);
 
+        DI_CHECK(z)
+
         arf_set_d(a, x.a);
         arf_set_d(b, x.b);
         arf_set_d(c, y.a);