diff --git a/arb_hypgeom/1f1_integration.c b/arb_hypgeom/1f1_integration.c
index d59cca35..50a9ace9 100644
--- a/arb_hypgeom/1f1_integration.c
+++ b/arb_hypgeom/1f1_integration.c
@@ -340,11 +340,11 @@ arb_hypgeom_1f1_integration(arb_t res, const arb_t a, const arb_t b, const arb_t
     arb_init(t);
 
     ok = arb_is_finite(z);
-    arb_sub_ui(t, b, 1, prec);
-    ok = ok && arb_is_positive(t);
+    arb_sub_ui(t, a, 1, prec);
+    ok = ok && arb_is_nonnegative(t);
     arb_sub(t, b, a, prec);
     arb_sub_ui(t, t, 1, prec);
-    ok = ok && arb_is_positive(t);
+    ok = ok && arb_is_nonnegative(t);
 
     if (!ok)
     {
diff --git a/doc/source/arb_hypgeom.rst b/doc/source/arb_hypgeom.rst
index f6a5bad9..b26b3d29 100644
--- a/doc/source/arb_hypgeom.rst
+++ b/doc/source/arb_hypgeom.rst
@@ -166,7 +166,7 @@ Confluent hypergeometric functions
 
     is used.
     This algorithm can be useful if the parameters are large. This will currently
-    only return a finite enclosure if `b > 1` and `b - a > 1`.
+    only return a finite enclosure if `a \ge 1` and `b - a \ge 1`.
 
 .. function:: void arb_hypgeom_u(arb_t res, const arb_t a, const arb_t b, const arb_t z, slong prec)
 
diff --git a/double_interval.h b/double_interval.h
index fed7e0ea..9e3a17ee 100644
--- a/double_interval.h
+++ b/double_interval.h
@@ -162,6 +162,8 @@ DOUBLE_INTERVAL_INLINE
 di_t di_fast_mid(di_t x)
 {
     di_t a, b;
+    if (x.a == -D_INF || x.b == D_INF)
+        return di_interval(-D_INF, D_INF);
     a = di_interval(x.a, x.a);
     b = di_interval(x.b, x.b);
     return di_fast_mul_d(di_fast_add(a, b), 0.5);